Rear Brakes Locking Up

[AuroraGirl]

Maybe mine is wrong, but it is as it came from the factory as far as I know, and the small line is hooked up to the front port of the master cylinder. I was always under the impression that the rear port is rear brakes, front port, front brakes. When I swapped in front disc on my 65 C20, I did it just like this and never had any braking problems.

Granted my 81 is a C20, I think the small line is supposed to go on the front port. Here is a pic of my C20 master cylinder...

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It depends on the master
Delco-Morraine vs Bendix vs whoever else they sourced them from, and the size of them

The low drag brakes have different masters than the earlier ones, and the ports are sometimes different

The combination valve in a split reservoir truck wouldnt have to block the flow, the leaky circuit will drain out and youll have one or the other

But in the gmt400 years, i believe they do that because of the shared reservoir, which only has a small dam that prevents fluid when low from all being consumed but would let it leak until then without the combination valve

 

[Blue Ox]

It could be either way. Some of the lighter weight trucks have the front brake reservoir and piston is in the back; heavier chassis trucks seem to have the front reservoir and piston in front. Usually, the fronts have the bigger reservoir of the two. Also, they use different size fittings to make it less likely to mix them up.

 

[legopnuematic]

Taylor beat me to it- and now Derek-

Master cylinders are not all the same, some have the large chamber in the front, some rear, some equal sized.

The large chamber should feed the front discs, as they use more volume for actuation/wear. The small chamber should feed the rear drums, as the wheel cylinders have much less volume/displacement.

For posterity-

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An 86 K30-

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And an 86 C20, to show different chamber configurations.

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To the OP, post a picture of your master. An 86 C10 should be an aluminum/plastic master, with the large chamber in front.

 

[Andronikos]

From the manual:

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Doesn't provide a lot of answers, but at least you can see what all is in there.

Thanks Blue Ox. The diagram reminded me I bought the tool that temporarily replaces the warning switch for bleeding purposes. It screwed right in no problem, so I think the valve is centered. I also checked the warning switch with a powered test light and it doesn't light up.

 

[Andronikos]

Maybe mine is wrong, but it is as it came from the factory as far as I know, and the small line is hooked up to the front port of the master cylinder. I was always under the impression that the rear port is rear brakes, front port, front brakes. When I swapped in front disc on my 65 C20, I did it just like this and never had any braking problems.

Granted my 81 is a C20, I think the small line is supposed to go on the front port. Here is a pic of my C20 master cylinder...

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Thanks DoubleDingo. I made all new steel lines for the truck and routed them the same way as it was when I got it. It might have been wrong all along. The guy I bought it from did alot of other questionable things to it.

 

[AuroraGirl]

I Think the trucks with combination valves which have the bypass are diagnolly split systems, that means it would probably be necessary or it would deplete the fluid
BUt the advantage is you get one front brake and one rear, which keeps you more stability -safe and more braking than just rears would

 

[Andronikos]

Taylor beat me to it- and now Derek-

Master cylinders are not all the same, some have the large chamber in the front, some rear, some equal sized.

The large chamber should feed the front discs, as they use more volume for actuation/wear. The small chamber should feed the rear drums, as the wheel cylinders have much less volume/displacement.

For posterity-

You must be registered for see images attach

An 86 K30-

You must be registered for see images attach

And an 86 C20, to show different chamber configurations.

You must be registered for see images attach

To the OP, post a picture of your master. An 86 C10 should be an aluminum/plastic master, with the large chamber in front.

Thanks Legopnumatic. I'ts aluminum with a plastic reservoir just like you describe.

 

[AuroraGirl]

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heres another master variation for the value of it

 

[Andronikos]

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heres another master variation for the value of it

Thanks AuroraGirl. I want to experiment by switching the lines at the master. Do you think I can get adapters? If not, I'll have to make new lines, not a big deal if I can't avoid it.

 

[DoubleDingo]

Thanks AuroraGirl. I want to experiment by switching the lines at the master. Do you think I can get adapters? If not, I'll have to make new lines, not a big deal if I can't avoid it.

I wouldn't mess around with switching them around. I would make certain that yours are correct first, and check/reset the proportioning valve first. It's a safety thing, and you want the best braking you can possibly have for the safety of you, your family, and everyone around you.

 

[AuroraGirl]

I wouldn't mess around with switching them around. I would make certain that yours are correct first, and check/reset the proportioning valve first. It's a safety thing, and you want the best braking you can possibly have for the safety of you, your family, and everyone around you.

ass stated before too the diameter is different, look at mine, my front one is larger diameter

THe wrong size wouldnt fit the opposite direction at all (too large and would be obviously incorrect)
The other way would be smaller and be evident for the opposite reason

Id imagine the valve only takes one of each size so its not able to be mixed

 

[Andronikos]

I Think the trucks with combination valves which have the bypass are diagnolly split systems, that means it would probably be necessary or it would deplete the fluid
BUt the advantage is you get one front brake and one rear, which keeps you more stability -safe and more braking than just rears would

I tested by deliberately locking up the tires with a good strong foot on the pedal, putting it in park, and getting out to see the marks. Only the rears lock up, both of them. The fronts left no rubber on the pavement at all.

 

[Andronikos]

Just my .02 from personal experience, but I would not change the valve as your replacement is going to be Chinesium. If you do change it , hang on to the old one. One thought might be that the valve is not centered. With a helper, try cracking the rear bleeder screws and push the brake pedal down and hold it until the screws are tightened. The idea is to get the valve to move the other way - if it is indeed off center.

When I do this should my assistant just simply push the pedal down as normal like we do when bleeding, or hit it fast, slow or otherwise? I'll try it this afternoon.

 

[Goldie Driver]

When I do this should my assistant just simply push the pedal down as normal like we do when bleeding, or hit it fast, slow or otherwise? I'll try it this afternoon.

Just slow, and hold it all the way down until both bleeder screws are closed. Be sure to make sure that the fluid level in the reservoir is OK before you do this and afterwards.

Don't want to run it dry. It's a free experiment if you have brake fluid.

 

[Andronikos]

Just slow, and hold it all the way down until both bleeder screws are closed. Be sure to make sure that the fluid level in the reservoir is OK before you do this and afterwards.

Don't want to run it dry. It's a free experiment if you have brake fluid.

Thanks. I'll do this when my "assistant" gets home. LOL