Bathtub Drainpipe Leak

[plutonic]

I've recently replaced the whole bathtub drainpipe from the overflow to the bathtub drain flange assembly. I used schedule 40 PVC pipe, and all has been well until I found a tiny leak, shown in the images below (I will try to get a video clip uploaded, but my file is too large).

I had to cut away some of the pipe coming out of the ground as it was cracked, so I used a slip coupling and an extra piece of pretty short PVC to extend that pipe up to the T fitting that connects the drainpipe and the overflow pipe.

I'm not sure if using a slip coupling was a good or bad idea, but that's where the leak is coming from.

^ the leak is on the left-hand side right above that yellow splotch.

I'm trying to figure out what my options are as I've come this far and want to make sure the solution is permanent. Here's what I've come up with so far (poll above):

 

[shim]

I am not a plumber, but I am an engineer. PVC glue works through a chemical solvent process that actually welds the two PVC surfaces together as if they were one. There is probably some kind of micro channel in the chemical weld caused by debris. I believe you could just brush on a bit more primer and glue there and that would reweld about a millimeter or so beneath the surface. More if you mixed up PVC shavings in there.

If you don't feel that is enough plastic holding back your leak I think you could try cutting a little bit from the rim off a fitting for this size pipe. And then placing it so the two clean cut edges of your little piece and the leaking fitting are interfacing. Then if you chemical weld it there there's a lot of plastic holding it back. And if the channel continues there is enough surrounding plastic to hold it back as well. I would make sure to sand and clean the surface beforehand, as you would not want debris to cause another micro channel in your weld again.

here is a picture of what I described: