can someone help me with my brakes?

[75chev4x4]

75 Chevy k20. For the last month or so my brake light on the dash has come on randomly while hitting the brakes. In the last week I have replaced the brake light switch bleed the brakes with a power bleeder and replaced my check valve on the booster. The problem has gotten worse. Even before doing any of this work when cold the brake pedal would got further to the floor and I thought this was because the choke was on and it would go away once warmed up. Now every third time I hit the brakes or so the pedal will do this and that red brake light comes on. To me it doesn't feel like air in the lines but like no power brakes? So now this red light is coming on when I brake not evertime but a lot. I read about the pin on the proportioning valve and it makes no sense to me and I'm not sure that this is even what is causing my problem. I rebleed the brakes again about half hour ago and still same thing. Was supposed to us my truck for camping Monday but I don't wanna risk it with this issue. Anyone have some advice on this?

[Prong]

Master cylinder is on the way out most likely. I've had 2 go bad over the years and this is what they'll do most of the time.

[HotRodScottsdale]

I had the same problem and mine was the brake booster,light came one every time. Hit the brakes and it felt like I was pushing a pillow with my foot, I replaced it but didn't bleed my brakes, I drove for a it and I had a air bubble to two but it kinda worked itself out, stil gotta bleed them but replacing the booster helped big time
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[75chev4x4]

Ok new info. I went out and rebleed each wheel the old fashion way had my dad pump the petal and I opened the bleeder. Got some air out of each one but the fronts would not evacuate all the air. Would be fine for a few lumps then boom bunch of air would come out. Cant seem to get all the air out of the fronts. Also read that u should test the wire at the prop valve with a test light to see if its shut. I don't have a test light so I used my my multimeter and it says 4.3 at the wire on the prop valve. Is the valve shut or is my mc booster going. God I bate this truck sometimes haha

[Axle]

Keep bleeding. Your dad is waiting for you to holler before letting up on the pedal?
Calipers installed with the bleeder on top?

Alex.

[75chev4x4]

Yea he tells me when the petal is down and I close it he lets the petal up and pushes down and I reopen.

[Desert1957]

Originally Posted by Prong

Master cylinder is on the way out most likely. I've had 2 go bad over the years and this is what they'll do most of the time.

Yes, Master Cylinder's can and will cause creeping pedal, and also can aerate the fluid if there bad enough.

Sounds like this is what you have , you bleed it , its OK for a while then the low pedal starts , making the light come on.

Make 100% sure there are NO hidden leaks , I have seen pin holes in rusty old brake lines that only leaked under extreme brake pressures. Even a seeping wheel cylinder can give you a crappy pedal.

Not an electrical problem , but a Hydraulic problem.

Desert

[75chev4x4]

I just bought a test light and tested the prop valve like someone else on here said to do and it makes a full circuit light comes on which says the valve is stuck? I thought mc leaked fluid when. They went bad?

[Desert1957]

Originally Posted by 75chev4x4

I just bought a test light and tested the prop valve like someone else on here said to do and it makes a full circuit light comes on which says the valve is stuck? I thought mc leaked fluid when. They went bad?

Not always, they can By-Pass fluid internally. causing poor pedal.

Desert

[Zebb]

Start with a fluid leak first to rule that out. Pump the brakes up and hold it. If it slowly goes down start checking all the lines while somebody is applying pressure. Check your wheel cylinders in the rear too. Hopefully it is something easy so you can get it fixed and use it this weekend.

[75chev4x4]

Pumped the brakes up with the engine off and they hold petal does not go down after I hold it.

[78 Big10 454]

Bad master cylinder is my vote.

[75chev4x4]

Should I just replace the mc then give that a shot. Nobody thinks its the pv stuck? Thanks guys.

[deth_1970]

how about a pic of the brake booster under the MC? any fluid there?

[75chev4x4]

Do u mean behind the mc? I haven't pulled it from the booster yet, but I don't see any leaking around the outside. Just got yo work but the mc is like $20 so when I get off I will go get the new one just ti eliminate that as a problem.

[75chev4x4]

It also seems like the truck being in gear effects the brakes. If I sit in park engine on I can lush the petal and its ok but if I back out and take it for a drive when cold if I hit the brakes right after getting off the gas it will feel like no power brakes. Not sure if its cause less vac because the throttle isn't closed yet? Just wanna fix this darn thing.

[75chev4x4]

Ok went to napa after work bought a brand new master cylinder. Came home bled it put it on the truck bled the lines on the truck twice brakes felt really good. Bled one more time took it for a drive and nice and tight brakes. No brake light and the pedal grabs up much higher than before. Wow brakes are tricky cant believe that's what was wrong with it. Thank you to all of u for ur advice it was very helpful. Get to take it camping after all. Thanks.

Nick

[Desert1957]

Good Job! , Glad to hear you bled the Master before installation.

Congrats on fixing your truck.

Desert