Brake Problem

[MtBraun]

I had to replace the proportioning valve and all the lines on my 78 K15 due to rot and a stuck valve in the PV. I bled everything to where I was almost happy with the brake pressure. Now, when the engine is running and brakes applied I lose a lot of pressure and the brakes will easily bottom out. When I turn it off and pump a couple times the brakes have good pressure. Is this caused by a leaky brake booster?

 

[1987 GMC Jimmy]

A bad booster will make a loud whoosh noise when you press the brakes. Like a snappy huff or a forceful sigh. I had that problem, but I don't remember the change in braking force. I didn't have a sinking pedal, but one of the master cylinder circuits lost all of its fluid with no leakage anywhere, plus I had that whoosh. I did diagnose it early before I really had a chance to play with the problem. Are you loosing fluid in the MC without leakage, or have you found any leaks? I'd go through and check your rubber lines, the caliper bleeder valves, and the backs of the rear wheels for bleeder valve or wheel cylinder leaks... You could also try plugging of the booster line and testing the brakes manually to see how they respond. I would be more mindful of the MC and the hydraulic aspect of the braking system if I were you because that's where all the action is. The booster just helps you out.

 

[MtBraun]

A bad booster will make a loud whoosh noise when you press the brakes. Like a snappy huff or a forceful sigh. I had that problem, but I don't remember the change in braking force. I didn't have a sinking pedal, but one of the master cylinder circuits lost all of its fluid with no leakage anywhere, plus I had that whoosh. I did diagnose it early before I really had a chance to play with the problem. Are you loosing fluid in the MC without leakage, or have you found any leaks? I'd go through and check your rubber lines, the caliper bleeder valves, and the backs of the rear wheels for bleeder valve or wheel cylinder leaks... You could also try plugging of the booster line and testing the brakes manually to see how they respond. I would be more mindful of the MC and the hydraulic aspect of the braking system if I were you because that's where all the action is. The booster just helps you out.

Not so much as a huffy child on this one. However I noticed the front side of the booster was wet underneath where the MC connects. I thought it was just fluid I spilled when bleeding so I wiped it dry and a half hour later it was wet. I haven't lost a lot of fluid but there is some loss. That suggests a bad MC to me.

 

[1987 GMC Jimmy]

I would say possibly or even likely since you have that plus the mushy pedal. I will say that I that the parts store lid was not sealing right to my MC (I know, shocker), and it blew fluid out in a pattern that looked much like what you're describing. I fixed the issue, cleaned it up, refilled it, and that was the end of it. However, I'm pretty doubtful that the same happened to you.

 

[highdesertrange]

sounds like you have a bad master. highdesertranger

 

[MtBraun]

Circling back to report I now have brakes that will lock up all four tires. I replaced the master cylinder and bled the brakes for what seemed like forever with a Mityvac. I did find a leak on the new PV connection, tightened that all up and they seem to be good to go.

 

[Honky Kong jr]

Good deal

 

[74 Shortbed]

Glad to hear..