my house is pretty old and still has the lead piping. the pipe looks like it started to crack under the sink and was patched with ROOFING TAR. and is now clogged. i tried plunging and the sink started filling with dust and grime and rust flakes. it started out with a small clog that would only take a few seconds for the water to receed. it is now staying backed up for close to an hour or so. im trying to decide between cutting out the messed up part of the pipe to replace with pvc, or to just rip out the hole lead piping(trying to do the cheapest fix for now) but the only problem is i dont know where the clog is. i havent yet takin the cap off the pipe in the basment to check if its clogged in there(waiting for water to receed enough).any suggestions? also if i go through with connecting pvc how would i go about cennecting pvc to lead pipe?