View Full Version : Troubleshooting a vacuum leak?

11-21-2005, 11:41 PM
How do you find/fix where a leak is in a vacuum system?

We are trying to set up a vacuum system similar to the one Bill P. talks about on his web page.

We have a leak somewhere. My partner has worked with compressed air system but never vacuum system. I have worked with neither.

11-22-2005, 12:34 AM
You have mail.

ron brown
11-22-2005, 09:39 AM

I will assume you are speaking the low-volume High-Vacuum system. There will be a lot of variables. ALL surfaces, backs and sides of ALL porous materials in the system must be sealed. A coat of paint is not nomally as seal ... or two or three. Two coats of good boatbuilding epoxy is, IMO, a minimum.

First test with a non-porous panel. Move the vacuum pump out of hearing range and listen for leaks. If you need to chase down the tubing start at the pump and work out the first time.

With that sort of system one is working on the thin edge of the process. ANY failure will show.

Good luck,


11-22-2005, 11:01 AM
If your checking the vacuum system? Did you use teflon tape? If you are sure that all the fittings are tight you might use smoke to help detect a leak. A punk stick will help you locate a leak. Ron is correct about starting with a non porous materal at the puck. Good luck.

11-22-2005, 01:17 PM
Take a length of tubing for the air pump in a fish tank and put one end to your ear. The other end can be used as a testor to listen for leaks around places. You will hear the leak(s) when you move the tube close to them.

Brady Watson
11-22-2005, 02:09 PM
Vacuum leaks on low vol/hi Hg setups can be a real bear to find. You will most likely not be able to tell where the leak is by simply listening for it unless you have a relatively large leak. While a bit pricey, an ultrasonic leak detector is the right tool for what you are looking to accomplish.


11-22-2005, 03:15 PM
All of the above points are valid. Sometimes it can be something as simple as just twisting a connection too hard, and you break a seal. Or there is a piece of sawdust in a fitting, or embedded in the gasket tape etc.
When Bob Standard and I first worked on that particular vacuum rig we both had a number of little "improvements"we had to develop along the way (i.e plastic fittings would leak, brass would not, etc.).
My best tool for finding these leaks has been the vacuum guage itself, as it lets you try all of the above possibilities until you see a definite change in the reading on your guage. Don't be afraid to disassemble the unit, and RE-assemble it completely. Sometimes it's the simplest of things that make all of the difference...

11-22-2005, 03:40 PM
Has anyone tried blocking off the system forward of the pump and presurizing to check for leaks using soap bubbles?

11-22-2005, 07:08 PM
Thanks for the excellent help, especially to Ted who talked things through with us this morning. You guys are the best.

The solution was to put a check valve between the pump and the vacuum gauge/rest of the system. The pump isn't design to hold a vacuum (called surplus center). The check valve solved the issue.

11-22-2005, 07:18 PM
Bob and I found that out as well, which is why I listed the check valve, and it's source
( www.veneersupplies.com (http://www.veneersupplies.com)) on the web page. As you can see from the picture it is the first fitting which comes out of the pump. Be SURE you have it oriented properly, as someone else bought a similar unit and apparently had connected it backwards and was having a lot of issues until he got it figured out...

Brady Watson
11-22-2005, 08:01 PM
Yep...no check valve will do it!

In case any of you other vacuum tinkering guys out there were wondering; yes a PCV valve from the local auto parts store will work just fine as a check valve for your vacuum rig.


11-23-2005, 07:41 PM
We have now built a system that sucks and we are really pleased about that.

I'm waiting for a few more pucks to arrive but we built one puck from scrap plastic and it works well.