View Full Version : Vacuum Chuck System – How I Built Mine

Steve Schlumpf
02-03-2008, 12:03 PM
While there is a lot of info on the subject of vacuum systems that can be located on the various forums, not many walk you through the process of putting one together. I researched vacuum systems, read every post I could on the subject and tried to get an idea of what it would take to put together an affordable system. While I am sure there are many different ways to construct a vac system, here is a simple system that works and cost less than $200.

The normal disclaimer: I have learned what I know through active participation here on SawMill Creek and trial and error. The following photos offer only a few of the many possible ways to hook up a vacuum system. Most importantly, if something doesn’t look or ‘feel’ safe to you – DON’T do it!

At this point I want to give credit to two folks who helped make my vac system become a reality:

Vaughn McMillan, (familywoodworking.org) from whom I borrowed the design of the cart, and

Tom Steyer, (sawmillcreek.org) who not only listed and sourced the individual components I needed but designed and built a custom rotary adapter to fit my Jet 1642 lathe. Thanks Tom! Without your detailed assistance my vac system would still be in the research stage! Tom makes adapters for several popular lathe models. You can
contact Tom at vacuumadapters@cox.net (vacuumadapters@cox.net) or through Sawmill Creek.

The following items were obtained from Surplus Center (www.surpluscenter.com (http://www.surpluscenter.com))

(In the form of Description, Item No., Qty)
Gast 0523 220VAC Vacuum Pump, # 4-1540, qty 1
Air Filter, # 4-1565, qty 2
Vacuum Gauge, # 21-1583-CA, qty 1
¼” NPT Brass Ball Valve, # 20-1486, qty 1
1/8” NPT Air Silencer, # 4-1657, qty 1
Bushing ¼”x1/8” NPTF, # 455-HH, qty 1

The following items were obtained from Menards:

3/8” ID x 20’ Vinyl Tubing, # 6840442, qty 1
½” ID x 10’ Vinyl Tubing, # 6840455, qty 1
¼” NPT x 3/8” ID Hose Barb, # 6801821, qty 7
3/8” NPT x ½” ID Hose Barb, # 6801847, qty 2
3/8” x ¼” Brass Adapter, # 6805940, qty 2
¼” Brass Elbow, # 6805827, qty 1
¼” Brass T - Female Threads, # 6805115, qty 2
¼” NPT Brass Barrel, qty 1
9/16” to 1” Hose Clamps, qty 10
20A/250V Standard Plug, # 3635362, qty 1
20A Double Pole Switch, # 3637535, qty 1
20A/250V Single Receptacle, # 3638974, qty 1
2” Rigid Casters, # 2171995, qty 2

I also picked up a Fram G2 fuel filter from Wal-Mart for less than $3. It’s used as an small air filter for the relief valve. There are a few other parts such as junction boxes, PVC conduit, outlet covers, etc that you can pick up at your local home improvement store as needed.

I wanted the vac system to be portable so that when finished using it I could store it out of the way of my lathe. I modified Vaughn’s idea of a cart/hand truck and used scrap pieces of lumber I had on hand. I am sure with a little thought and planning you can come up with something that fits your needs.


Building the System

I installed the Gast pump such that the input/output were oriented to the left which placed the pump wiring close to the wall of the cart and out of the way. The pump was mounted to the ¾” plywood using ¼” bolts and lock washers so it wouldn’t vibrate loose.

On the front side I installed a junction box (for power) at the location I wanted the pump wires to come through the wall. A ¾” hole was drilled through the junction box and plywood wall for the wiring. A second junction box (for the on/off switch) was then installed directly above the power box. I installed a short piece of ½” PVC conduit that I just happened to have left over from installing the 220 VAC drop for my lathe.

For the power hookup I used pieces of an old 50’ 16-3 outdoor extension cord. I cut a piece about 12’ long and installed the 220V standard plug on one end and routed the other end through the pump side of the wall and into the power junction box. From there I threaded it through the conduit and into the on/off switch box. A 3’ piece of the extension cord was then run from the on/off switch box back through the conduit and into the power junction box.

The on/off switch (220VAC double pole/single throw) was then wired with the long power cord connected to the input side of the switch and the short 3’ piece connected to the output side of the switch. Pulled all the excess wiring back down into the power junction box, installed the switch in the box and attached the cover plate.

Next, using wire nuts, I wired the Gast pump to the 3’ piece of orange extension cord. Made sure all the connections were secure - then installed the cover. If you have to ask how to wire the pump to a 220 VAC line – PLEASE get someone qualified to make the connection for you! While I have no problems showing you how to make point to point connections on the vacuum lines (worse that can happen is you lose vacuum), if you hook up the 220 VAC wrong and it could get serious!

This is where the fun begins as there are any number of ways you can route the vac lines. A simple overview of what we are going to do - connect the lathe to the vacuum pump, provide a filter to keep the wood dust out of the pump, install a gauge to monitor the vacuum and provide a means to regulate that vacuum.

Starting at the lathe we have to use a special vacuum chuck. These need to be different sizes and can be either bought or made in your shop. I will give an example later of a simple homemade chuck that works great.


In addition to the pump you need an adapter that provides a means to attach the vacuum hose to the spindle. There are a number of adapters out there that fit different lathes but I decided to go with the adapter Tom Steyer designed as it allows me to just plug it into the handwheel without having to bolt it on or buy yet another threaded adapter. I also love the fact that when I am done using the vac system I simply pull the adapter from the handwheel and wheel the vac system away!

Step 1: Connect the rotary adapter to the input air filter using the 3/8” ID vinyl tubing. I used about 4’ of tubing but the amount is up to you.

Remember to use pipe tape for all your threaded connections!

I installed a ¼” NPT elbow at the adapter in order to route the vinyl tube away from the lathe. I then connected a ¼” to 3/8” barb to that elbow so I could connect the vinyl tubing.

The air filter has a ¼” NPT input connection but a ½” output barb – don’t ask me why they are different cause I don’t know. I installed a ¼” to 3/8” barb on the input and slid on the vinyl tubing. I actually used a hair dryer to warm the vinyl tube before sliding it on the barb and it helped a lot! I then secured both tube connections with small hose clamps.

Steve Schlumpf
02-03-2008, 12:04 PM
Vacuum System continued…


Step 2: Secure the air filter to the 3/8” plywood wall. I figured where I wanted the filter and drilled a few holes through the plywood and held the filter in place with tie-wraps. Real Hi-Tech!

Step 3: Connect the output of the air filter to vacuum relief valve. The output barb of the filter is for ½” ID tubing. I heated the end of the ½ vinyl tubing, pushed it on the barb and secured it with a hose clamp. I then moved the tubing around until I found an area I wanted to place the relief valve and vac gauge. I cut the tubing at that length leaving just a little extra.


Step 4: Connecting the ½” tube to the rest of the system required an adapter that went from a ½” barb to 3/8” NPT. Then an adapter that went from 3/8” NPT to ¼” NPT. The ¼” NPT threaded into one end of the first ¼” T.


Step 5: Connect the Relief Valve to the T and then attach the small air filter to the bottom end of the Relief Valve. The small air filter is to prevent dust from entering the system when regulating vacuum. I also used the ¼” rubber hoses that came with the filter – just because they look really cool!

Step 6: Connect the ¼” barrel to the T and then connect to the remaining ¼” T. Attach the Vacuum Gauge as shown and then attach another ¼” NPT to 3/8” barb so you can connect to the 3/8” ID vinyl tubing.

Step 7: Run a length of tubing from the ¼” T to the input of the pump. The input and outputs of the pump are labeled on that end of the pump. Cut the tubing to length and install a ¼” to 3/8” barb into the pump. MAKE sure you attach the 3/8” vinyl tube to the input side. Secure with a small hose clamp.

Step 8: You are going to connect some 3/8 ID tubing to the output of the pump but first you must figure out where you want the output air filter located. Drill a few holes and secure the output filter with tie-wraps.

Step 9: Install a ¼” NPT to 3/8” Barb to the output side of the pump and another one to the input of the air filter. Cut a piece of 3/8” ID vinyl tubing to length and install between the pump and air filter. Secure with hose clamps.


Step 10: Determine location for Silencer and attach to the structure. Connect the output air filter to the Silencer using ½” ID vinyl tubing and adapters as listed in photo.

At this point the construction of the vacuum system is complete. All you need now is a vacuum chuck!

Steve Schlumpf
02-03-2008, 12:04 PM
Making your own Vacuum Chuck

The following is a simple vacuum chuck and is only one example of some of the materials you can use for chucks.

Step 1: The wood is rough cut cherry approximately 1” thick and the PVC is a 4” coupler. I surfaced one side of the cherry, cut the piece in half and glued the pieces together using Elmer’s Ultimate High Performance Glue. It’s one of those glues where you wet the surface of both pieces of wood, spread the glue and then clamp it overnight. The glue expands and fills all the small areas between the two boards that could potentially cause a vacuum leak.


Step 2: You can use a faceplate to mount the vac chuck to your lathe but I use the Beall Spindle Tap and will demo how I use it to make my own faceplates.

Mount the glued up wood between centers and round the outside such that it can fit into the jaws of your chuck. I have a Talon chuck and use the larger #3 jaws to hold the blank for the next step.


The outside is now rounded over.


Step 3: Mount the blank in your chuck and bore a hole 1/8” less than the finished tap size – meaning 1/8” less than the size of your spindle. In my case I drilled a 1 1/8” hole all the way through the blank. There is enough room for the forstner bit to clear the chuck once it cuts through the blank – however – it is a good idea to watch closely and not ruin your drill bit!


Step 4: Following the instructions for the Beall Spindle Tap, I start the tap into the wood and have the back of the tap supported with the tailstock. The spindle of the lathe is locked so that the blank cannot move. I use a crescent wrench to turn the tap a ½ turn and then snug up the tailstock center. Repeat until the tap is fully inside the wood and the tailstock is no longer needed to help keep the tap traveling straight. Tap the wood all the way through. Unlock the spindle!

Steve Schlumpf
02-03-2008, 12:05 PM
Making a chuck continued…

Step 5: Remove the blank from the chuck and then remove the chuck from the lathe. Thread the blank onto the spindle and true up both faces of the blank.

Step 6: The idea here is to turn a shoulder area that will support the PVC coupler. You want to make this a fairly snug fit and should take your time when you get close to the final size.

I dry fit the PVC often until it makes contact on the shoulder as well as the inner tenon area.


Once you have a good dry fit I clean up the shoulder area with a parting tool – just to make sure I have a good 90* angle between the shoulder and the inner tenon.

Note: make sure the PVC is clean and it’s edges are smooth so as to make good physical contact with the wooden faceplate.

Step 7 (no photo): Glue the PVC coupler and the wooden faceplate together. I use hot melt glue and finds it works very well. I run a bead of glue along the shoulder/tenon junction and then seat the coupler while the glue is still hot. I then run a bead of glue along the inside of the chuck at the PVC – wood joint and also along the outside joint. The glue cures very fast but I usually let them sit for an hour or so before returning the chuck to the lathe.


Step 8: With the chuck mounted on the lathe, use a bowl gouge and true up the PVC and round the edge over. Use sandpaper to clean up any tool marks.

Also, I highly recommend the use of a closed cell foam gasket on the rim of the chuck to help provide a good seal and protect the turning. Any number of different closed cell materials can be used. You can find 1/8” craft foam (with self-adhesive back) at Hobby Lobby, Michael's, and other craft stores. Use white or light colors to avoid possible stains on your work.

Congrats – you just made your own vacuum chuck!

Remember, there are many different ways to achieve the same results. The steps I’ve listed here are simply what I did to build my system. As we are all learning, please feel free to ask questions, offer suggestions or comment.

For those interested, I have made a PDF copy of this tutorial – please PM me with your email address and I will forward you a copy.

Bill Bolen
02-03-2008, 1:09 PM
Steve that is a great how-to! The steps are explained very well and the photos add a lot of clarity for each step! This one is as good as your hollowing how-to! So, are you looking at a new profession! On-line how to"s.Great job and thank you for the excellent tutorial...Bill...

Andy Hoyt
02-03-2008, 1:44 PM
Great presentation, Steve.

Where the heck was this thing a few years ago when I needed it!

PS: This thread will eventually get moved into the Articles Forum, but for now it's right here.

Tom Sherman
02-03-2008, 1:51 PM
Great info Steve, and a great job of assembling/presenting it.

Bill Stevener
02-03-2008, 2:17 PM
One of the better ones Steve, Very nice, very informative, nice photos.

Scott Donley
02-03-2008, 2:19 PM
Very well done Steve. With your words and pic's, I may even attempt one. Thanks !

Bob Justin
02-03-2008, 7:10 PM
OK, Thats it! I've put this off for about a year and a half now. You show pictures of my lathe so I had to place the order this morning with the guys at surpluscenter for the vacuum pump. Looks like I'll be busy putting this little gem together shortly.

Great how-to instructions along with clear pictures that show the details.

One note on teflon tape, keep it back off of the first thread so it can't get into the air path after assembly.

Ben Gastfriend
02-03-2008, 8:25 PM
Wow! Yet another tutorial! Really nice description. I'll repeat what I said in the thread about HFs.... I'll store this for future use!:D

Bernie Weishapl
02-03-2008, 8:29 PM
Thanks Steve for the information.

Tom Hamilton
02-03-2008, 9:08 PM
Thanks, Steve, another turning mystery unraveled. Thanks for the detailed explaination.

Now, have you worked out the details of supplying these to the Creeker's who prefer to have an original made by the inventor? I'll take unit number # 2, as you obviously have claimed unit # 1.

Paypal or Credit Card?

Congrats on a great tutorial, Thanks, Tom

Hilel Salomon
02-04-2008, 8:52 AM
Steve you are a wonder!!! Not only is the ARTWORK you create fantastic, but your generosity with your time is truly great. I really appreciate this tutorial.
Thanks Again, Hilel.

mike johnston
02-04-2008, 9:11 AM
Toms adapter is a great idea, Just remember if you have a power matic lath there can be some variation in the ID bore in the spindle. so measure carefully. Tom takes great steps to insure a propper fit.


robert hainstock
02-04-2008, 10:19 AM
Your usual very thourough job. Thanks for al you give to the Creek and others, and thanks for a;; you do for me. :D:D:D:D:D:D

Bob Justin
02-04-2008, 8:50 PM
Just too outstanding of a project illustration not to copy.
A few questions . . .:)
Any thoughts to adding a small expansion tank and vacuum switch so the pump can cycle off and not run at 100% of use time?
A common manifold to supply to a laminating bag or maybe making a set up for stabilized wood processing?
Possibally one of the acrylic flowmeters with a brass control valve for fine control of air flow?

A very nice clean design from the original to the recent revision with so many possiblities beyond just the vacuum chuck.

Dennis Peacock
02-04-2008, 9:33 PM
Most excellent writeup Steve!!!!!!!!!!!!!!!! :cool:
You did GOOD my friend. ;)


I've talked with a couple of folks in the vacuum industry. They state that it's best for a lathe vacuum chuck setup to be a "full time run" type setup. This is primarily due to the woodgrain allowing air through and into the vacuum setup. I'm not saying it can't be done, but you will run into times where, no matter what, you'll be running the vacuum pump full time to keep the bowl or platter "stuck" to the lathe chuck. DAMHIKT!!!! :)

Jon Lanier
02-05-2008, 1:03 AM
Steve that looks great. Only problem is... that was way over my head. I'm way to dyslexic to follow... I need to see it being done. Want to come down to Ohio and show me how to do it? :)

Jeff Cremers
02-05-2008, 10:34 AM
very nice job

Fred Conte
02-05-2008, 11:01 AM
Thanks Steve for taking the time to create this presentation - excellent and professionally done!


Ron Drew
02-05-2008, 11:09 AM
Fantastic Steve. Appreciate the time it must have taken to put all that together. Your tutorials are the best. Ron

Scott Donley
02-13-2008, 10:58 PM
Great presentation, Steve.

Where the heck was this thing a few years ago when I needed it!

PS: This thread will eventually get moved into the Articles Forum, but for now it's right here.Just a reminder before it gets lost :D

Chuck Jones
02-25-2008, 9:42 PM
I have finished building my vacuum chuck based on your design and instructions... except I'm waiting on delivery of the adapter from Tom Steyer. The plans were fantastic. I did not have one bit of trouble... except in my part of the world there is no such thing as a 3/8” NPT(MIP) x ½” ID Hose Barb. It was quite amusing how many parts I ended up with to make those two connections.

I assumed the pump from The Surplus Center would be used. It looks like a brand new out of the box rig to me. I'm also amazed at how quite it is. I may have wasted $6 on the silencer. But come to think of it, I'm pretty deaf. For my set up I don't need any where near that much vinyl tubing. Anyone looking to save a few bucks might want to do a little figuring ahead of time, or find another project to use the excess tubing.:D

Many thanks for your effort and for sharing. Great set of plans!

Steve Schlumpf
02-25-2008, 10:49 PM
Chuck - I'm glad you found the tutorial helpful in building your system! You're going to love using it!

I understand the issue with the 1/2" Hose Barb - I also had to try and figure a way to mate the vinyl tubing off the air filter to the rest of the system. Glad you were able to find something that worked! Up here the vinyl tubing I used comes in rolls and you either get the 20' or 10' roll. I agree that leaves a lto of extra tubing. I just rolled mine up and stored it in case I decide to try vacuum veneering someday.

Again, glad you were able to build your system with minimal hassles! Have fun with it! Looking forward to seeing some of your work real soon!

Bob Justin
03-04-2008, 10:58 PM

Once again great instructions! Like most, I chose to modify things a bit by removing some of the hose and the hose barb joints. A bit rusty on my tubing bend layouts, but it works just fine. 20 years worth of parts laying around just waiting for a plan to assemble. So now I am set up for either full vaccum or a regulated flow via a flow meter. The surplus vacuum pump with one minor scratch is the part that put the whole plan together. Again, thanks.:)

Tom Steyer
03-06-2008, 3:31 PM

Glad that you got your system built so quickly. Your adapter has been shipped. Sometimes deliveries take a few days longer for some orders, just due to the production cycle. Our adapters are usually shipped within two weeks of receiving your order - often more quickly. Sorry for any inconvenience to you.

Chuck Jones
03-07-2008, 1:34 PM
I got my adapter from Tom Steyer today. Everything came together and the vacuum chuck works fine. This "Chuck" has a new learning challenge. How the heck to get things centered. I've had that problem with every method of re-mounting I've tried. I think part of my problem is I glued the foam to the rims of the chucks with hot glue. I've never used hot glue much before but I'm discovering that it hardens pretty fast and I think the glue is throwing things off a bit. Gonna listen to my wife now and not use hot glue when I redo the chucks. She said the foam would stick fine without it. We'll see.

Tom Steyer
03-09-2008, 10:35 AM

Try just using the self-adhesive "craft foam" sheets available at Hobby Lobby, Michael's, or most any craft store. They don't stick real tightly - I usually have to press them back down after they sit for a while, but I'm still using the original pieces on my chucks after many uses.


Larry Rupert
03-09-2008, 3:38 PM
Really nice job, start to finish. I saved it as a .pdf for my future reference as well. Thanks for posting it. Larry

Allen Neighbors
03-09-2008, 6:59 PM
Steve, thanks for the excellent tutorial... and the time and energy you put into it.

Grant Davis
03-10-2008, 7:46 AM
EXCELLENT tutorial Steve.

Steve Schlumpf
03-20-2008, 2:59 PM
I’ve been getting a few emails from folks that are in the process of putting together their vacuum systems. I think that is great and know you will love the system once operational!

Wanted to let everyone know that it was politely brought to my attention that the air filters used in my system were actually installed backwards. According to the specs - the ¼” NPT opening is actually the output and the ½” barb is the input.

While I did disassemble each filter before placing it into the system so I could inspect the element and also make sure the filter was seated tightly against its O-ring, I saw nothing in the housing or on the element that would suggest air flow was unidirectional.

So, a simple test was called for.

I wheeled out my vacuum system and turned it on to find out what the vacuum was with the air filters in the original configuration. All I did was plug the end of the lathe adapter with a piece of plastic bag - the first reading was 24 hg. To determine if the air filter routing made a difference with regard to the air flow, I then disconnected the input filter to my system and tested for hg. It measured 24 hg. I then disconnected the output filter to make sure it wasn’t causing backpressure to the system and again tested for hg. It again measured 24 hg. So, about as un-scientific as you can get but I don’t believe the routing of the air filters change anything.

While this new information does not change the operation of the vacuum system, in my opinion, it nevertheless needs be presented so each individual can decide if they want to rig things differently or not.

There is one design change, as a result of this information, that would be worth considering and that concerns the exhaust air filter. If installed ‘correctly’ the Silencer/Muffler would then screw directly into the filter housing. That would clean up the exhaust area by eliminating some hose and a couple of brass adapters. Course, you would then require a different adapter to connect the ½” barb of the filter to the exhaust port of the pump. It’s always something, eh?

Hope the info wasn’t confusing. If you have questions, please let me know and I will do my best to answer them.

Richard Madison
03-20-2008, 6:57 PM
Somehow I missed the reason for the exhaust filter. Is the vacuum pump oil-lubricated? Certainly looks like a dandy system.

Steve Schlumpf
03-20-2008, 10:10 PM

The vacuum pump uses graphite vanes and over time they will wear down. The filter was placed in-line to prevent that dust from blowing all over the shop.

Richard Madison
03-20-2008, 11:42 PM
Thanks, Steve. Did not think of that. Realized that I have a small piston-type compressor that I modified some years ago for use as vacuum pump, but never actually used it as such. Even if it works as originally intended, would need a surge tank and some baffles to smooth out the pulses. Something to try in spare time. Can always buy a real vacuum pump if it comes to that.

Chuck Jones
03-21-2008, 9:44 AM
I actually wondered about the input/output routing when I was building mine. I wasn't concerned enough to do the scientific test you have now performed, but I reasoned that going from smaller diameter to larger was probably to prevent back pressure in the filter. I also figured if it mattered the manufacturer would label the ports. Made sense to me when I was in a hurry to get mine working, but I'm not sure it does now.

In any case mine works great and I'm not messing with it. (Besides I don't want to have to make another 3/8” NPT(MIP) x ½” ID Hose Barb transition.):)

Steve Schlumpf
03-21-2008, 10:07 AM
Chuck - I got an email from member Bob Justin and I hope that he doesn't mind but I would like to quote him with regards to this matter:

"My understanding for most filters has always been that the "Dirty" air or liquid should enter on the larger surface area of the filter and the smaller surface or clean side should route to the pump. For this application, I would want to have the visual in the filter housing as larger chips and dust build up so it can be cleaned as needed."

I also received an email from the person who first brought this to my attention and his concern - having not seen the filter - was that it could collapse under vacuum if it was designed for directional airflow. The test I ran and a description of the filter element (similar in construction to the reinforced paper filters used on Shop-Vacs) have alleviated his concerns.

So, the air filters will work in the system regardless of their orientation. I personally like the input filter installed the way it is - so I know when to clean it - but I may end up changing the output filter just to clean up the exhaust area a little bit.

Hope this helps.

Chuck Jones
03-21-2008, 10:14 AM
I've made a few of the chucks according to your plans. The first couple have too much wobble. I think the hot glue cooled too much before I got the pvc coupler seated. On the next ones I seated the pvc without the glue, then ran a bead of glue inside and out. They are much better.

I built a slight variation that has proved very useful for small items like boxes and lids. It's built the same way you describe up to the point before turning the shoulder for the coupler (through step 5). Instead of using the coupler, I simply covered the entire piece with the foam (except for the tapped hole of course). I drew some concentric circles with a sharpie to help with alignment. I'm amazed how well this holds small items with a flat surface or smooth rim, except when I forget to close the regulator valve, which has only happened twice now.

Thanks again to you for the plans, and to Tom Steyer for the adaptor.

denis strods
05-21-2008, 3:19 AM
Thanks for a great post, i learnt a lot from this.

David Haywood
07-25-2010, 8:58 PM
Steve, I have seen a system (home built) that incorporated 2 short pieces of 4" PVC placed in-line to act as a vacuum reservoir in cases of lost power. Are you familiar with such an item and what do you think of it's usefulness?

Steve Schlumpf
07-26-2010, 5:30 PM
David - have not heard of that particular version but sounds like it would work just fine. So far - knock on wood - I haven't worried to much about losing vacuum but things like that do happen!

David Woodruff
09-23-2010, 6:52 PM
Steve, I have seen a system (home built) that incorporated 2 short pieces of 4" PVC placed in-line to act as a vacuum reservoir in cases of lost power. Are you familiar with such an item and what do you think of it's usefulness?

Same thing as a pressurized tank in reverse. Instead of 120 psi positive pressure resevoir, you have a resevoir of 25"-28" Hg vacuum, mostly nothing in the tank except for a few stray molecules.. So the very maximum vacuum is equal to atmospheric of about 14.7 psi at sea level acting upon the surface of your tank. Quiet a difference compared to 120 psi of internal positive pressure. So why worry about a vacuum resevoir failing. Anyway which is worse and implosion or explosion? I don't know, never had either.