my problem is this. the 3/4" water line from service to my water heater (hot water line) is leaking. the top part of the pipe is pvc, then the 2 nut, pvc connecter, and into the copper pipe that leads to my water heater. i purchased a new 3/4" X 3/4" coupling, followed the instructions, and turned back on the water from outside. i still have a leak at the top of the coupling where it is pvc. i took everything apart and reassembled it, making sure that everything was tight, but it still leaks. any advice?