Rear Brakes Locking Up

[CalSgt]

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

 

[CalSgt]

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]

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]

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]

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]

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]

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]

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]

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]

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