Rear Brakes Locking Up

CalSgt · 2025-04-30T16:38:01-0400

Also, did you verify the piston size of the replacement master cylinder?

CalSgt · 2025-04-30T16:41:28-0400

The computer I'm on wont let me edit a post so here's another...

Please post a picture of your front rotors/calipers from the back side
Please post a picture of the rear assembly with the drums removed

Andronikos · 2025-04-30T18:35:24-0400

Goldie Driver said:
Well, let's assume the valve is good. Maybe a silly question, but are the longer shoes on the rear ? Rear is the secondary shoe position which is the longer shoe.

Yes.

Andronikos · 2025-04-30T18:39:59-0400

Old Guy Bill said:
There is always some confusion between “Combination” valves and
“Proportioning” valves…
Get a Wilwood valve similar to this one, make sure of the fitting size.
Mount and plumb it into the rear brake line, and adjust it to where the rears don’t lockup.
This is very popular in circle track racing.

You must be registered for see images attach

I have looked at these and I'm fine with it. I have some questions on the plumbing. Simply add this like you said with no other changes? I can leave the existing plumbing as is?

Andronikos · 2025-04-30T18:45:43-0400

CalSgt said:
A few clarifying questions:

Did the problem exist before replacing everything?

if not did you verify your new wheel cylinders and calipers had the same diameter pistons as the originals?

A) No idea. I replaced all the existing stuff with new parts the way the old stuff was assembled. B) The new stuff is stock replacement parts and looked just like the old stuff. I don't take wheel cylinders apart to verify size.

PrairieDrifter · 2025-04-30T18:59:58-0400

Check to see if your rears are dragging during normal driving. Go drive like a mile without much braking and see if the drums are hot.

I fought rear brake drag when I did my brake job. Changed the master and brake pads and shoes, nothing else. Ended up with brake drag. Nothing fixed it but driving it, I still don't think it's 100%. But the drums aren't 400 degrees anymore.

83Stepper · 2025-04-30T19:48:05-0400

Before getting all crazy and reconfiguring things and cutting into stuff, have you tried jacking up the front end and with the help of an enlisted friend, try and spin the front wheels while they hold the brakes? This will tell you if you're building any pressure in the front brakes. Also are you certain that you've adjusted the rears correctly? If they're too far out, you'll be dragging the rears and can potentially cause the rears to lock prematurely before enough pressure is sent to start applying the front brakes.

It was always my understanding that pressure was sent to the fronts first, then residual pressure was sent to the rears from the valve to assist in slowing down, hence the reason that even on my 77 C10, the original shoes were still installed after 48 years. Yeah they needed to be changed only because the left rear wheel cylinder popped and made a mess

AuroraGirl · 2025-04-30T19:51:01-0400

Andronikos said:
I have checked them five times. Seriously, five times. Everything is "perfect".

i disagree, if everything is checked five times , then it couldnt be the case unless your tires are grossly different in pattern size and stickiness

AuroraGirl · 2025-04-30T19:52:47-0400

PrairieDrifter said:
Check to see if your rears are dragging during normal driving. Go drive like a mile without much braking and see if the drums are hot.

I fought rear brake drag when I did my brake job. Changed the master and brake pads and shoes, nothing else. Ended up with brake drag. Nothing fixed it but driving it, I still don't think it's 100%. But the drums aren't 400 degrees anymore.

brake hose diameters swell in and restrict, uncle tony showed one that had a pin hole diameter to through it
And was like wow, but when they do this they are not sound structurally, any reverse of the pressure in the system might check valve the line to stop the fluid and thus hold pressure

AuroraGirl · 2025-04-30T19:56:05-0400

83Stepper said:
Before getting all crazy and reconfiguring things and cutting into stuff, have you tried jacking up the front end and with the help of an enlisted friend, try and spin the front wheels while they hold the brakes? This will tell you if you're building any pressure in the front brakes. Also are you certain that you've adjusted the rears correctly? If they're too far out, you'll be dragging the rears and can potentially cause the rears to lock prematurely before enough pressure is sent to start applying the front brakes.

It was always my understanding that pressure was sent to the fronts first, then residual pressure was sent to the rears from the valve to assist in slowing down, hence the reason that even on my 77 C10, the original shoes were still installed after 48 years. Yeah they needed to be changed only because the left rear wheel cylinder popped and made a mess

the backs get pressure first, the to overcome springs and begin their self-energiziing design, then the fronts have less distance to move to apply, and once the rears were beginning the fronts are getting the pressure and they begin braking, so it feels balanced
Then the pressure is mostly in the front because it needs more ,but the rear is kept mathematically below a point that a normal situaiton would see them lock, so you have as much braking as possible without a way to modulate the pressure depending on load or conditions like ABS and rear load proportioning do

AuroraGirl · 2025-04-30T20:00:18-0400

Andronikos said:
CaA) No idea. I replaced all the existing stuff with new parts the way the old stuff was assembled. B) The new stuff is stock replacement parts and looked just like the old stuff. I don't take wheel cylinders apart to verify size.

so you said you checked everything 5 times, but you verified you replaced things as a parts canon and did not inspect things up close

So you are lying to us or maybe us and yourself

You needed to replace ALL the parts? To the brake pedal pad, to the drums/discs at the other end, to the booster vacuum filter and hose to the intake you REPLACED EVERY component?

please make a list of them if you did not quite literally do that
words have meaning and every single part of the system has a specific purpose, and some of them I think of would affect things you might not even realize means that you may be missing something
If you have a list on a website like amazon you could send screenshots too

Otherwise actually seeing your brakes like @CalSgt says is my vote on next thing (plus tell us what everything was)

CalSgt · 2025-04-30T22:14:02-0400

Andronikos said:
A) No idea. I replaced all the existing stuff with new parts the way the old stuff was assembled. B) The new stuff is stock replacement parts and looked just like the old stuff. I don't take wheel cylinders apart to verify size.

The first question was asking if the rear brakes were locking up before the work was done, did you not drive it before?

The reason for the second question was “stock replacement” could still be wrong parts, due to different options many incorrect parts can still bolt right on and sometimes even work just fine. If you still have the old parts somewhere don’t turn in your cores or toss them until you get to the bottom of the issue.

CountKrunk · 2025-04-30T23:48:49-0400

Thread needs more pics

Rear Brakes Locking Up

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