Troubleshooting Vacuum Leaks in Veneering Bag Systems

      Pros advise on how to keep a vacuum bag veneering setup air-tight. July 28, 2006

I have constructed a vacuum press. It has two additional canisters that feed additional vacuum into the bag to minimize the cycles to complete the pressing. When testing the system (without any bag attached and the line blocked), I can pull 27Hg, but the pump recycles within 4.5-5 minutes.

I have resealed every connection, trimmed and replaced every hose connection and clamp. I have run the system and used smoke to detect where the leak might be and have yet to find one. The pump is new. Any ideas how to detect the leak?

Forum Responses
(Veneer Forum)
From contributor T:
I think 27 Hg is so much vacuum that it's bound to leak! Does it leak at 20 Hg? Mine shuts down at 21 Hg and restarts at 16 Hg, averaging 18-19, and it presses well. 4 or 5 minutes between cycles seems good to me. Try applying fresh sealant around all the fittings for a minute while the vacuum is high and then release some vacuum so it doesn't get pulled out again and let the sealant cure before using it again. Maybe the check valve has a slight leak, but I think your system is good to go as is. It is leaks in the bag that are going to drive you crazy, and they are usually at the bag opening in the corners or at the vacuum hose connection. I am finally confident with my homemade kit, bought venturi system after about 10 pressings on paying projects. It's a real moneymaker for me. My next system will use a pump, so large leaks won't matter!

From contributor K:
If your shop is quiet, you should be able to hear the hiss of small leaks by holding your ear close. If the leak is a small hole in the bag, those are easier to find by feeling the wrinkles. The closer you get to a leak, the softer the wrinkles will be. Then run your hands over the surface, and listen for the faint hiss to stop. Duct tape is a good quick fix here.

From contributor E:
One thing to check is your vacuum pump. Some pumps have a small hole in one of the valves. This is to equalize the pressure in order to make it easier for the pump to start up. If this is the case, you need to put a check valve between the pump and bag. Easy fix.

From the original questioner:
Update: I gave up looking for the leak and made a 2' x 3' bag from 30 mil poly and pressed a gorgeous quarter match marble walnut panel. It turned out well.

The system is set to run up to 20Hg of vacuum and then shut off. After 2Hg drop, the pump cycles back on. The 4-5 minute cycle time is now down to 15-30 seconds. I have located and repaired any leaks I found in the bag, but suspect the hose to bag connection.
I have a top line check valve installed and all fittings, connections, and canisters have three coatings of silicone followed with duct tape. I am at the point where I am going to start over with the installation and test each and every component as it is installed and correct any leaks before moving forward to the next component. At the current cycle rate, the pump will wear out 10-20 times faster.

From contributor T:
Try separating the bag from the rest of the system first by installing a shutoff valve in the vacuum line at the bag. This way you can isolate the bag and watch if it loses vacuum. I'll bet it's the bag closure. After clamping between cauls, I inject latex caulk into the corners of mine to get a final seal, then wipe it off later. Also, put a strip of foam sill gasket (see your building supplier) between the vinyl in the open end before closing. The bag connecter I use is from and works well. Looks like a tire stem with a stem clamp from an air pump, high quality parts.

From contributor E:
Vacuum bag systems with lots of fittings can always be a problem. It sounds, though, as if you are close to figuring out the problem. Here are a couple of thoughts that may help.

I agree that sealing the open end of the bag is probably the problem. Cauls are always a problem as you never get them just right while in a hurry to close the bag. The bag material when folded over and clamped can always leak. It takes just a little crease in the material and it's game over. The other problem with cauls is that you cannot see where the leak is because it's covered with wood. I use a butyl tape in our kits (Roarockit) that, when applied to the sealed end of the bag, allows you to actually see where the leak may be. There are a couple of different types of this material you can buy online if you do a search. If there is a local Woodcraft store close to you, they also have it as a replacement component for our kits. C channel type systems are also a problem and can also leak, especially at the seams of your bag. The sealing tape we use is very aggressive and there is an acquired technique you need to learn to open the sealed bag. Check valves. I bought an expensive one once and it leaked.

My rule of thumb with vacuum veneering is, keep it simple. The fewer joints and connections you have, the better. Now, some people may not agree with me on this one, but if you have a good vacuum bag, check valve and regulator, there is no need for a reservoir. These only add more connections that can leak and really do little to maintain constant vacuum pressure. If you have a leak, it's going to drop in vacuum, reservoir or not.

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