Rear brakes lock up on occasion

[AirSpeed]

I replaced all the brakes on my 72 GMC 1/2 ton 4X4, new pads and wheel cylinders in the rear and new calipers on the front. On occasion my rear brakes will lock up at a stop and I can feel them drag sometimes as well. It doesn't happen all the time, just on occasion. It did this even before I replaced all the brakes. I checked all the components in the rear and they look fine, I'm wondering if it could be my proportioning valve? If bad, can it cause conditions like this? Can it cause pressure to remain in the rear cylinders after the brakes are released? Is it repairable or should I replace it? What else should I look for?
Thanks for any suggestions!
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[1969~C-20~Heavy-Half]

Same thing happened to me after I had all my brakes replaced and fixed. its your master cylinder. You either need a new one, or you have a bad seal around the top gasket of the Master Cylinder.


1969 C 20 Heavy Half

[leddzepp]

With no weight in the bed, it is not uncommon for your rear brakes to lock up...happens to me a lot and I have all new brake components including the master cylinder.

[AirSpeed]

Originally Posted by leddzepp

With no weight in the bed, it is not uncommon for your rear brakes to lock up...happens to me a lot and I have all new brake components including the master cylinder.

I would agree except mine seems to drag for awhile when this happens, they never drag unless they have locked up at the last stop. Once they lock up and I try to accelerate and can feel the brakes holding still, if I pump them it will usually release.
My master cylinder looks like it may have been on it for the entire 39 years so it can't hurt to replace it I guess! I checked my local NAPA and they have a rebuilt one for $35. For that price its worth a try!
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[1969~C-20~Heavy-Half]

Im telling you its far worth the try becaause I had the same issue. Truck would stop fine just the back brakes would lock up and stay locked sometimes, I would try it out and if not then atleast you have a new master cylinder?

[Longhorn Man]

the "staying locked" part makes me think of the hose from the frame to the axle.
A rubber lid gasket has absolutly no direct effect on the braking performance in any way shape or form. The only way it can indirectly effect them, is if all the fluid leaks out.

[ls1nova71]

Sounds like the classic proportioning valve/ meterind valve problem to me. It's job is to hold off the rear brakes for a split second so the fronts come on first. Id replace it, they're not serviceable.
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[68gmsee]

I agree with Longhorn man... sounds like the typical collapsed rubber hose symptom. Pressurized fluid is forced through the collapsed hose but there's not sufficient spring force to push it back to the master cylinder so the wheel stays locked for a period of time. Usually, after some time it will seep back and the wheel will be free again until the next time you step on the brake.

If this is what's happening, raise the rear wheel up and have someone step on the brakes and release them. Try turning the tire by hand. If it's locked up, release the pressure by using the bleed valve. If that loosens it up, then more than likely you have a collapsed rubber hose. It's on top of the rear axle passenger side on 1/2 ton two wheel drive trucks.

btw: you can't tell it from the outside.

[Stricklinator]

Something similar happened to me and it turned out to be a bad rear brake hose. I replaced it and it hasn't happened again.

[Longhorn Man]

Originally Posted by ls1nova71

Sounds like the classic proportioning valve/ meterind valve problem to me. It's job is to hold off the rear brakes for a split second so the fronts come on first. Id replace it, they're not serviceable.
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that's not how the proportioning valve works on these trucks. All it does is restrict/reduce the line pressure going to the rear. Doesn't delay the fluid so the front brakes work first.

[CrashBob]

Same here, I rebuilt/replaced the whole system and the rears kept locking up when I backed up. oldgold70c10 suggested the parking brake could be the issue, I loosened it a bit and no more lock up!

CrashBob

[custom1970]

For what it's worth, i agree with everything Longhorn has said. It's very common for the rubber to swell and the hose act like a check value. Besides, if the hoses are 40 yrs old, they should be replaced anyway just to be safe.

[stich626]

Originally Posted by Longhorn Man

the "staying locked" part makes me think of the hose from the frame to the axle.
A rubber lid gasket has absolutly no direct effect on the braking performance in any way shape or form. The only way it can indirectly effect them, is if all the fluid leaks out.

x2
the rear rubber line is failing from the inside,
not uncommon

[AirSpeed]

Originally Posted by stich626

x2
the rear rubber line is failing from the inside,
not uncommon

I understand the theory but would that account for the fact that when I tap on the brake peddle they release instantly?
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[AirSpeed]

I went ahead and bought a brand new master cylinder, I'm going to install it tomorrow. I have never replaced an entire master cylinder, it looks like a simple job but I remember hearing it should be bench bled, is this necessary and what is the process? When I replaced all my brakes I remember needing to tape the end of the proportion valve in order to get the rears to bleed properly, is there anything else I need to keep in mind while changing the master and bleeding the system? Thanks for any tips or suggestions!
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[68gmsee]

Originally Posted by AirSpeed

I understand the theory but would that account for the fact that when I tap on the brake peddle they release instantly?
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Just one teensy bit of info that you forgot to mention.... lol

If they release quickly then you could be on the right track. MC, prop valve or brake booster if you have power brakes.

[AirSpeed]

Originally Posted by 68gmsee

Just one teensy bit of info that you forgot to mention.... lol

If they release quickly then you could be on the right track. MC, prop valve or brake booster if you have power brakes.

I did mention that, in my second comment. I bet I end up replacing the entire system! I've replaced just about every other bolt on component on this truck! Thanks!
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[Boog]

Gear oil leaking onto the brake shoes from a bad axle seal will make the rears lock up easily too.

[Longhorn Man]

but that wouldn't make them stay on and drag though.

[Boog]

Depends on how sticky they are.

More than likely its' a combination of things.

[68gmsee]

Originally Posted by AirSpeed

I did mention that, in my second comment. I bet I end up replacing the entire system! I've replaced just about every other bolt on component on this truck! Thanks!
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Yeah, I saw that on the other post but I didn't know it was a quick release. When I had the collapsed hose on mine it would release slowly. This happened around 15 yrs ago and there wasn't a lot of info or help available on the net. I wound up replacing a lot of parts, too.

When you find out what it is, let us know.

[stich626]

rear brake rubber line, you tap the brakes and it lets go..why, well when you stop the bracks have x amount of psi , say 500psi you let off and the pressure try's to flow back to the master, and blocks the line,(it collaps inside) , you tape the brakes with a fast tap and it opens the line, but the pressure in the lines isn't high enough to colapse the rubber line again so it flows back to the master, untill you stop again..

[AirSpeed]

Originally Posted by stich626

rear brake rubber line, you tap the brakes and it lets go..why, well when you stop the bracks have x amount of psi , say 500psi you let off and the pressure try's to flow back to the master, and blocks the line,(it collaps inside) , you tape the brakes with a fast tap and it opens the line, but the pressure in the lines isn't high enough to colapse the rubber line again so it flows back to the master, untill you stop again..

Sounds reasonable to me! I am going to go ahead and replace all of them tomorrow, the NAPA in my little town should have them, I hope! I know they make hydraulic lines any length, I'm pretty sure they can make brake lines easy enough!
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