1968 Mustang rear brakes locking up

mcqueen

Member
Jan 6, 2022
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Castle Rock, CO
Hey There,
New to Stangnet! Finally cured my proportioning valve leak and ran a new brake line all the way to the rear end. I bought the car with non-power disc/drums (which may have been an incorrect setup to begin with) and have since added a power booster and new MC. Brake lines have been bled appropriately and I road tested her last night. Under medium brake pressure my rear drums are locking up, obviously not good, especially if I have to ever really stomp on the brakes. I bought my brake booster and MC from Master Power. From searching on this forum I have learned that all drum brakes require a residual valve. Apparently I need a 10psi residual valve mounted after the proportioning valve and anywhere before the rear brake line splits to each wheel. What does a residual valve do? Does it limit the amount of pressure from the MC to the rear drums giving the front discs priority to do their job of stopping the car?

Thanks in advance for any help/advice!
 

WORTH

15 Year Member
Nov 18, 2002
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The residual valve is not the issue, does the master you bought have a large and a small reservoir? If it does the large is for the front brakes if you have disc, and the small is for the rear. Is that the way you have it hooked up? Is the proportioning valve you bought adjustable?
 

mcqueen

Member
Jan 6, 2022
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Castle Rock, CO
I knew I didnt cross my lines up, but I just went and checked under the hood. Large reservoir to front brakes, smaller reservoir to rear brakes.
I did not replace the proportioning valve, and it is not adjustable. Could the car have the wrong proportioning valve? Is there a specific proportioning valve for front disc/rear drums?
 

WORTH

15 Year Member
Nov 18, 2002
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Either you have an issue with the rears, or maybe a mismatched master cyl. Check the rear brakes, make sure they are clean and the e-brake cables aren't too tight. And don't adjust tem too tight, if you still have an issue, I'd buy an adjustable Pro valve. And try that.
 

mcqueen

Member
Jan 6, 2022
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Castle Rock, CO
Worth, when the rear brakes lock, they both lock at the same time, so at least I can keep the car pointing straight down the road. Today I adjusted (loosened) the e=brake cable and also removed the rear drums. There was quite a bit of dust in there, but definitely no leaks and no contaminants on the shoes. I also took 10-clicks off each side (i know that will return with continued braking) but I want to see how it brakes work on the next test drive. To slushy on the roads today to get it out. I'll report back.
 

Ranger

Active Member
Jun 10, 2013
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give Master Power tech support a call and explain your dilemma and they will put you back on the right track - safely
a residual valve maintains brake line pressure in a disc/drum setup ...
the more I learn, the more I realize how little I know
 
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manicmechanic007

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Sep 26, 2017
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Backing off ten clicks might be a bit much
Might lead to a low pedal feel
Try just a few clicks at a time
And the contaminants like mcqueen mentioned are a big deal
So is the drum surface and the diameter
I replace all that most every time because I can get the parts so cheap
Inspect the parking brake cables for binding
(the shoes will be cocked sideways on the backing plate if the E brake cable is not released all the way)
 
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dennis112

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May 15, 2005
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I don't think that it was mentioned but a kit comes with 2 shoes that have longer pads than the other 2. The short padded shoes should be facing the front of the car and, of course, that means the longer padded shoe should face the rear of the car.

Might want to post a photo of each side and post it here.
 
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I don't think that it was mentioned but a kit comes with 2 shoes that have longer pads than the other 2. The short padded shoes should be facing the front of the car and, of course, that means the longer padded shoe should face the rear of the car.

Might want to post a photo of each side and post it here.
I will check how the shoes are installed, crazy if it was something simple like that.
 
Backing off ten clicks might be a bit much
Might lead to a low pedal feel
Try just a few clicks at a time
And the contaminants like mcqueen mentioned are a big deal
So is the drum surface and the diameter
I replace all that most every time because I can get the parts so cheap
Inspect the parking brake cables for binding
(the shoes will be cocked sideways on the backing plate if the E brake cable is not released all the way)
I'll take it to 5 clicks. Thanks for your input!!!
 
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dennis112

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Reread your first post and wondering if your proportional valve is adjustable? Might need to adjust it for less pressure to the rear brakes. If yours isn't adjustable then consider adding one if the current problem doesn't reveal itself. That's commonly done when people swap from drum to disc front brakes to cut down the pressure to the rear drums.

Also agree with Worth that it has nothing to do with not having the 10lb residual valve. Its purpose is to maintain some pressure on the rear drums to basically take up some slack that is inherent in drum brakes. Right now, that is the least of your worries but you should file it for possible future needs.
 
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I don't think that it was mentioned but a kit comes with 2 shoes that have longer pads than the other 2. The short padded shoes should be facing the front of the car and, of course, that means the longer padded shoe should face the rear of the car.

Might want to post a photo of each side and post it here.
Hey Dennis112,
I pulled the drums this morning to check placement of the shoes and the larger shoes are facing the back of the car on both sides as you said they should be. My proportioning valve is not adjustable but I just ordered a new one that is definitely disc drum. I think my car may have started life as a drum drum car. When I bought it there were discs up front but no power booster and no braking issues. When I added the power booster, and new Kelsey Hayes disc brakes up front, that's when the problems started (locking rear brakes). Once I get the new proportioning valve installed I will get back to you with the results.
 
Hey Dennis112,
I pulled the drums this morning to check placement of the shoes and the larger shoes are facing the back of the car on both sides as you said they should be. My proportioning valve is not adjustable but I just ordered a new one that is definitely disc drum. I think my car may have started life as a drum drum car. When I bought it there were discs up front but no power booster and no braking issues. When I added the power booster, and new Kelsey Hayes disc brakes up front, that's when the problems started (locking rear brakes). Once I get the new proportioning valve installed I will get back to you with the results.
It makes sense that the bias would be different with all those changes--it could negate the balance that you once had.
Installed the new disc drum proportioning valve today, bled system and road tested. Rear brakes are still locking up, but seemed to do so less and less as I drove around the local high school parking lot. I think I will give it a couple more local drives to make a final assessment.