SpudFiles

So the cannon that I am in the process of building is not big or fancy, just making it to chuck tennis balls for the dogs. The problem is that one of the two tanks leaks air out of a pinhole leak in one joint. I can dissassemble the tank from the valve and fill setup but I can't get to the joint because the tank is all glued up already.

I understand that if I can create a vacuum inside the tank it will pull cement into the hole and plug it up from the inside. Does this really work?

Is my regular household vacuum going to do this or do I need to find and borrow a big shop vac or even something bigger?

See Pic for you visual learners. Thanks for all thoughts.

we used to use that system to find fuel leaks on aircraft. we were basically "sucking" dye from the external of the aircraft and seeing where it came in. this didn't require much vacuum if i recall. we were inside the tank so a lot of vacuum wasn't healthy.

it should depend on what your using to seal the leak and how big the leak is.
the smaller the leak the harder it is going to be to seal it.

I should have added that the leak is found and has a big Sharpie "X" on it. It is small leak, I somehow managed to get one even with twisting when I assembled the thing. I could probably enlarge the leak with a pin or needle or something if that might make it easier to fill.

buy some jb weld and fill it with that it works on piston valves

Are you saying use JB weld on the exterior of the joint only or create a vacuum and try to suck jb weld into the hole, because if I remember right that stuff is thick?

Has anyone had this happen to them and how did they fix it?

I have sealed holes just by pasting it on, but I'm sure using a vacuum would work better.

Your household vacuum will work, especially if the rest of the chamber is sealed. It won't take much, though. As far as JB Weld goes, the stuff is pretty thick and it depends on the size of your hole. Just go with the same kind of glue you used on the pipe.

just pasting it on works for me but you way might work better

I apply jbweld on the outside it works making it as thick as posible, if you want the more sound but much harder way with the vacum you can use some of that high impact marine grade epoxy jack always uses so much its thinner, but if you want to thin out some of your epoxy you can add acetone or buy long term project type resin epoxy it's usually thinner

i don't think you should use actual vacum you should just presurize the chammber and rotate it so the leak is on the bottom just becareful to not damage the valve

So, my household vacuum worked liked a charm. The hose extension matched the connection for the valve perfectly and definitly created a vacuum since I am lucky to have an extra guage around here that reads negative (for use on skidloaders somewhere). My regular old upright vacuum hose was ducttaped onto the threaded adapter the valve attaches to. The vaccum created around a negative 8-10 psi, according to above guage, stable vacuum after around a minute. The variance is due to the vacuum works harder at times. It has one of those clean/dirty sensors on it to adjust how hard it works.

I applied the PVC cement with a syringe first right into the leak, then with the brush over top that a couple of minutes later. There was a noticible change in the sound of the vacuum cleaner when I applied the cement. I left the vacuum run for around 10 minutes after I first started applying cement to get a complete fill. Then I untaped the vacuum hose from the tank to let the tank dry at ambient pressure leaving the valve port open so it can breathe and vent.

I will be hooking up the valve and pressurising the tank this evening to see how well it worked. I will post results.

oh so thats what you meant with the vacum I'd still recomend real epoxy rather that pvc cement

So, the regular old household vacuum worked like a charm. It sealed the hole with no problem. I would suggest leaving the vacuum running for around 10 minutes even though the cement sets up real fast.

I think the syringe application helped because it got the cement right into the leak. Ater that set up, I went over it on the outside.

I charged the tank up yesterday and it went to 75 psi with no problems and held it for 30 minutes with only about 2 psi lost in half an hour. I can't complain about that.

Hope this helps anyone else with a small leak after the glue up.