Advice on how to seal test plug

I'm air pressure testing my PVC DWV lines on a new house. I'm having a devil of a time getting the test plugs to seal in the drain opennings. The 4" work fine, but the 2" ones - for the w/d, tub, shower, etc. are all leaking. I'm using the simple wing-nut variety from HD. I've cleaned the plugs and opennings well. Tightened them well. They did sit the winter unheated, indoors, at the site. Could the rubber be "dry"?

Any advice? Get new plugs? Put vaseline on the rubber? Wrap with teflon tape?

Thanks

Air testing pvc with compressed air is very dangerous and is a big no no. If the pvc pipe should fail the plastic parts will become shrapnel. You probably are wanting to use air so you don't have water in your lines over winter. In that case just drain the water out after the test where the line is stubbed out below the ground for your septic or sewer hookup. I glued a cap with a threaded thingamabob in it so I could screw a faucet to open and close so I could drain the water out if I had a leak. When I went to attach the septic I just cut the cap off and hooked up the line. Make sure you blow out any p-traps that are in the wall or below the floor with compressed air so they do not freeze. This is not sealed so it should not be a danger.

I did a no no and used compressed air to test the cpvc fire sprinkler pipe which like I said is a big no no. No one was allowed in the house while I was testing. The only exposure I had was a glued on endcab and about 4 inches of 1 1/2" cpvc. I used safety glasses. It was a risk that I needed to take because I could not get all of the water out of the fire sprinkler system. It is a wet system. It was going to stay unheated through the Winter for 4 years. It's one of those do as I say not as I do things. I did use water for the dwv system though. My dwv system is made out of abs and I used abs glue to glue the test caps on. These are thin white plastic but the abs glue still worked. I used those wherever I could. they did not leak. I did have trouble with the expansion plugs specially the 4" for the toilets. I finally got them to seal. Some are made better than others. The red plastic ones from HD appear to be disposable (they aren't priced that way though). Do you have anybody that has plumbed a house that has some good metal ones?

I use air to test PVC dwv all the time...quite rare that I do a water test. In my part of the country it is an accepted testing process.

The mechanical plugs you've mentioned arent my favorite for plugging off piping but I do have to use them from time to time. The problem that I've found with these plugs is that in order to get a really good seal, you have to put your channel locks to the wingnut....most often breaking off a wing. What I like to use for piping coming out of the wall is a simple glue cap. Just stub out of the wall 6-8" or so and glue that puppy on. (a rubber quick cap works just as well and can be reused). No worries then. I wouldnt put any sort of lubricant, especially a petroleum based lube, on the rubber. The petroleum will deteriorate the rubber and if you use a different kind of lube the air pressure is just going to push the mechanical plug out. Another thing you could use would be test balloons. These can get a bit pricey for a single use.

One thing you didnt mention was how much air pressure you're putting in the system. Here we use 5psi...no more no less.
Cherne Clean Seal Plug

Thanks

Thanks. Building inspector wants to see 5 lbs. I tried "moistening" the rubber on the plugs with silicone. Didn't help. Finally went to the the hardware store and got the rubber quik-caps you described and they worked fine.

I used the first method you described for all my sinks - roughed in with 6" or so extra pvc pipe and added a glue on cap. I sure wish I'd done the same for the other five drains. Next time.......