I'm having some braking issues on my 77 Corvette. It has power brakes. The master appears to be the original unit, the proportioning valve is much newer. The pedal is not soft, it feels about the right firmness and goes about 75% to the floor max. But the brakes are just too weak. The car stops too slowly. It's impossible to lock up the wheels and skid no matter how hard you press the pedal.
No leaks. All lines are in great shape - no rust. The sensor on the proportioning valve is not grounded (meaning the fluid flows to the front AND the rear).
I cracked the bleeder valve open on the front passenger side caliper, hooked up the whole clear hose & bottle set up and tried bleeding it. The first 3 or 4 pumps, the fluid came out briskly and quickly filled up the clear hose. But after that it slowed to a trickle and every subsequent pump on the pedal only forced a tiny amount of braking fluid out. I think that's the issue.
I also checked to see if the sensor on the proportioning valve goes off as I press the brake pedal (suggesting it restricts flow to the front), but it doesn't.
I'm out of ideas. What could be the issue? Master cylinder?