I finally got the new line in the rear end of my '79 Catalina today, and it doesn't leak anymore. While bleeding brakes, I noticed the master cylinder still forces fluid upwards into the reservoir somehow, the best I can describe is it looks like a straw is blowing up through the liquid, splashing it out of the top, but I can't really see any actual bubbles. The pedal is still mushy and there's no sign of another leak. I replaced the master cylinder with a remanned unit from Advance Auto last weekend, which I had a feeling might be a mistake. The original cylinder had the same issue. Fluid still slowly seeps down the front of my booster when I press on the brakes, so I assume the remanufactured unit is also bad. Is that weird fluttering of the fluid in the reservoir also a sign of the MC being bad, or is that from something else, possibly a bad booster? I found out tonight that Amazon carries brand new ACDelco master cylinders for the car for $13 less than the remanned unit from the auto parts store, so the remanned unit is getting returned.

Here's a picture of the front of the booster that I took before the remanned unit got installed. How do you tell if fluid is getting down into the booster? I checked that black rubber seal at the bottom, and it doesn't look like brake fluid is getting past it. The booster's functionality seems fine -pumping brake with engine running builds pressure to almost normal feeling, but it slowly bleeds off. When the engine is off, pumping the pedal a few times firms it up until it has only a little bit of give, which I'm assuming is related to the leak/ fluttering fluid.