Technical Brake guru's

Problem: brake builds pressure and locks up front disc brakes on 1937 Buick Special.
Tried so far:
New rubber lines (twice)
New master cylinder with factory residual pressure brake line seats COMPLETELY REMOVED

ZERO inline residual pressure valves. Did have 2# to discs and 10# to drums.
Rod from pedal to booster has 1/4" free travel
Rod from booster to MC has 1/4" free travel
When cold pedal is only 1/3 of its travel off floor
Kit is from Scarebird and uses Riviera rotors and Astro calipers.
Brakes are perfect for first half dozen stops and then build till pedal is solid with no free travel and drag old straight eight engine down to a stand still.
Only part not replaced is 7" dual diaphram booster.
System worked fine for thirty years with drum/drum configuration.
HELP!

 

My brother's 57 Pontiac once did this. After checking everything I finally removed the master cyl and put it on the bench.........found the return port in the master had some gunk in it. Cleaned it out and voila! Worked fine then.

 

Is something heating up while you drive for a while? Is there a line near the exhaust somewhere?

 

You’re losing 1/2” of travel in the pushrods and it still works?

Is it locking both wheels or only one? Crack a bleeder, see if it releases.



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I'd start loosening things to find the last place the pressure releases. How did you figure the 1/4" between the master and booster?

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Did you try contacting Scarebird? He is usually pretty good on follow up help.

 

Wheel bearing too snug? Not allowing piston to be pushed back in caliper.

 

Is the caliper sticking and not moving back in so the pad stays on the rotor?

 

what do you have to do to get them to release? does opening the bleeders solve the issue?

 

This symptom is most often caused by the piston in the cylinder not returning and therefore blocking the port. This is often a result of the push rod adjustment not being correct. Re-check your free-play setting but I'd start by backing the push rod off completely (no engagement of the push rod by the pedal arm) to allow the piston to return.

- EM