85 C3500 brake issues

[brothers69c-10]

So heres the scoop. I was drivingmy 85 crew cab last week and noticed the brake pedal fading more and more. I checked the reservoir and the front compartment was empty and the rear was full. There has to be a leak somewhere so I checked all the lines from front to back and they were all dry. I replaced the right side line that runs along the rear axle anyways as I did the left side a couple months ago. Filled the reservoir and bled the brakes. The first pumpgoes clear to the floor, bit will pump up on the 2nd and 3rd pump. We bled the brakes for 30 minutes and nothing changed. We were bleeding the brakes at the lines because the bleeders were rusted stuck. I decided to change out the wheel cylinders so I could bleed the brakes appropriately. Did that and after bleeding for quite awhile still the same results. Next decided to change the master cylinder. Bench bled that and bled the brakes again. So basically everything has been replaced and I cannot find any leaks anywhere. So when you go to stop the pedal at first has a little bit of pressure and you feel the brakes grab right away and then let go and you can continue topush the pedal down clear to the floor before they grab again. Yet you can pump them a couple times and then they're like normal. I should add that this truck has hydraboost. Anybody have any ideas?

[Rascal_one2001]

I have had my 85 for years and always has issues with my brakes and could never get them to bleed correctly so I set pans under each bleeder port and let gravity bleed for 30 min while topping off fluid and worked for me. I also recently replaced master cylinder and front calibers and now they bleed great.

Are you still losing fluid after your replacements?

[brothers69c-10]

I haven't drove it enough to find out. The last loss was over a 3 month period. That was the last time I checked it anyway.with the engine running, I can pump the brakes up and hold them for a couple minutes and the pedal won't go down at all. I'm just baffled with this.

[brothers69c-10]

I should also mention that it has, what I call it anyway, a manually adjustable proportioning valve on the rear axle. It bolts to the frame and has an arm that attaches to the axle itself. I read it works based on the amount of weight on the truck so basically ride height. Could this be bad and causing my issues?

[83GMCK2500]

I think if that rear load-dependent valve fails it would just restrict the flow (or not at all depending on where it failed) and cause you to brake unevenly front to rear.

Here is a recent thread about it. In this thread it is mentioned WHY GM added it, what their intentions were, and the service bulletin that THEY published to remove it.

[brothers69c-10]

I think I'll try removing that this weekend, just to eliminate one more variable and see what happens.