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Front brakes locked up

62440

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l have 70 miles on my car and it has been good until now. It has the 76 Diplomat front disk conversion with no leaks. Driving up to a stop sign l take my foot off the gas and car slows down, put the brakes on and front wheels lock up. I cracked the front line and got the car home. As l pull into the garage it locks up again. The car has been a long build and sitting dry for quite some time. I'm thinking the rubber has gone bad. I can't find a mopar performance 2 bolt alumimium MC locally and only one at Summit Racing. There was only one review and the guy said the ports were too deep to be able to tighten the fitting. I think my MC was a 1 1/16" but the brain is getting fuzzy. My pedal is high and hard. The one at Summit is 1.031" l believe. Is that a compromise size between 15/16" and 1 1/16" only thing that worries me is that bad review
 
Manual or power brakes?
Either way it sounds like the equalizer port is being blocked not allowing the brakes to release...
 
Push rod between pedal and master isn't set correctly and won't allow full master piston retraction or the pressure to bleed back to master reservoir.
 
Push rod between pedal and master isn't set correctly and won't allow full master piston retraction or the pressure to bleed back to master reservoir.

Otherwise known as the equalizer port is blocked..
 
Manual brakes, if it was the pushrod it should of showed up right away me thinks.
Push rod between pedal and master isn't set correctly and won't allow full master piston retraction or the pressure to bleed back to master reservoir.
 
Manual brakes, if it was the pushrod it should of showed up right away me thinks.

Is the pedal returning fully to the top.. Any chance the brake light switch is slightly pressing on the linkage...
 
I did have a problem with the brake light switch. But wouldn't that show up right away
 
I did have a problem with the brake light switch. But wouldn't that show up right away
If the switch is set right at the edge of holding the pedal under pressure & the switch binds ever so slightly it could cause the pedal to not fully return... I had a customer car do exactly that a few years ago...

FWIW the problem you describe is classic blocked equalizer port... The way it works is as long as fluid can flow through the equalizer port the brakes are released... As soon as you depress the pedal a slight amount the piston in the M/C moves & blocks the port so it can begin building pressure.... If the seal falls the pedal will sink, thats whats happening when you have a bad master cylinder & the pedal slowly sinks, the fluid is bypassing through the equalizer port...
 
Thanks, you may have saved me some $$$ I'll try a look at it tomorrow if l get a chance
 
Sitting dry... as in no fluid in brake system ....ever ?What exactly is "the 76 Diplomat disc conversion" ? Some kind of "kit" that you bought ? Some of the parts off of a Diplomat ? All of the brake parts from a Diplomat ? Just spindles and rotors ? brake hoses ?master? lines and proportioning valve ? Off of running car or junkyard ? We aren't there, help us out or people here will just guess and assume things and throw out all kinds of answers. More details get better results.
https://www.allpar.com/history/mopar/brakes/disc-main.html
 
Sitting dry... as in no fluid in brake system ....ever ?What exactly is "the 76 Diplomat disc conversion" ? Some kind of "kit" that you bought ? Some of the parts off of a Diplomat ? All of the brake parts from a Diplomat ? Just spindles and rotors ? brake hoses ?master? lines and proportioning valve ? Off of running car or junkyard ? We aren't there, help us out or people here will just guess and assume things and throw out all kinds of answers. More details get better results.
https://www.allpar.com/history/mopar/brakes/disc-main.html

Diplomat brakes are FMJ spindles, they aren't my first choice but they work fine....

Why does everyone seem to love kits? Most of them are junk, I'd much rather work with OE parts.....

Thought I will grant you brake hoses ?master? lines and proportioning valve? are all good questions... But FWIW the equalizer port is standard on all master cylinders, if it gets covered the brakes will drag or lock... So it is a very valid point to check early in diagnosis....
 
A dr. doesn't diagnose treatment based on just " it hurts" . There are a lot of valid guesses but still just guesses. We don't know what parts or their condition , since it was not factory installed that we are dealing with. The best rule of diagnostics is to never assume anything works or is correct. Start from scratch and check everything, one thing at a time and then move on to the next thing. That way you will know what the actual problem was. All components must be in good serviceable condition to start with. Get yourself a book/manual on brakes if you are dealing with a brake system. Do you do brake servicing on a regular basis ? The brake system is the heart of your cars safety. Ask someone who is there, that can see what you have and has brake experience to help you. Safety first, always.
 
All parts are new. I crawled under the dash and as best l can tell the brake light switch looks good, the pedal moves about 1/4 " from the top and the push rod looks like it has about 1/8" free play. I found the part # for the MC and it is the 1 1/32" bore. Looking at the Mopar Performance catalog and it states the 15/16" MC is for cars "Under 2800 pounds" and the 1 1/32" is for cars "Over 2800 pounds" My Plymouth should be about 3200 pounds there abouts I'm guessing, its been on a pretty strict diet. . I can't find the installation instructions and l can't remember the clearance for the push rod. The car ran great for the first 70 miles
 
To use the 1 1/32 master you need a booster...

Back in the 70’s Mopar didn’t sell a car other than an A body with disc brakes without a booster..

But people have been building them that way for years...

If you use the larger bore without a booster you need very strong leg muscles & likely still won’t be able to lock the wheels
 
Thanks, I'll give the 15/16 a try. How much clearance should there be between the pushrod and the MC ? I'm thinking contact and backed off a bit
 
Thanks, I'll give the 15/16 a try. How much clearance should there be between the pushrod and the MC ? I'm thinking contact and backed off a bit
That’s what I do when setting one up
 
I ordered a 15/16" MC from Dr Diff. I'll let you know how it works out. Thanks for all the replies
 
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