Brake woes

[bob8619]

1968 C10, drum brakes all around and theyre not power brakes brakes.

I've been having major issues with trying to get my brake pedal to firm up. Bled the system 5 times with 4 quarts of fluid but no fix. I noticed the fluid level in the reservoir for the rear brakes was slowly dropping so I looked everything over many many times but I find no leaks.

I started suspecting the master cylinder so upon closer inspection I noticed that the fluid in the front brake reservoir level goes up, as if the rear fluid is being forced into the front reservoir. I figured this was a master cylinder issue so I replaced it. Bench bled the master, installed it and have a rock solid brake pedal. Hooked up the front brakes only and bled them, less solid but not spongy by any means. Hooked up the rear brakes and bled them and mushy pedal again. Then I noticed the reservoir levels are doing it again, lower in the rear, higher in the front. This only seems to happen when they are both connected to the brake lines, when the rear brake lines were disconnected the fluid level doesn't budge.

There is a proportioning valve in there, could that be letting fluid feed into the front reservoir?