PROPORTIONING VALVE LEAK
I had one his leak there too. When I spoke to him he said to just throw a wrench on it and tighten more than you think is necessary until it kind a chirps (ie is very tight) and it should stop. It did.
On another note though I found that there isn’t much proportioning on it. The rears in that car will lock up before the front. In fact I can’t get the front to lock up at all.
Its his 2 post master 15/16 bore, his 73 and up A body one piece pro valve, new Kelsey 4 piston brakes, and oem 2"w x10" rear drums. Not that happy. Going to change out to oem pro valve and oem big bolt pattern stuff soon on that car…