brake troubles

:( HELP!!!! I just finished putting power brakes on the old `47 Coupe. I used an underfloor mount, with a `90 Mustang booster/master cylinder combo. It has drum brakes both front & rear. I used a 10# check valve both front & rear with an adjustable proportioning valve in the rear line.
My problem is that the front brakes keep locking up. Sometimes only after driving just a few blocks, but sometimes I can drive a hundred miles with no trouble. I keep bleeding the system, but I'm at a loss as to what else to do.
Can anyone help with this problem?

Along with what Streets mentioned, you might want to check the length of the rod that runs from the pedal to the booster to make sure there is enough play. If it's too tight it will not allow the piston to return. But then again, maybe I'm wrong. Go figure.:whacked:

Since you say "brakes" I'll assume you mean both front brakes are locking, not just one side. And, again assuming, they don't stay locked, it's just that the fronts skid and the backs don't.

Not sure which brakes you have, the original Lockheeds or later Bendix's from, say a Ford pick up. Did you change the shoes or just do the M/C-pedal swap?

I don't understand why you put a proportioning valve to the rears. With drum front and rear that's not necessary, and if you have it adjusted to reduce the rear brake action then that's probably why the fronts lock first.

Streets' comment would be compounded by your restricting flow to the rears as hydraulic levereage is a function of M/C piston size vs wheel cylinder size.

thanks guys for the input. additional info: `53 f-100 brakes front & `74 comet brakes rear. both fronts stay locked up until I crack the bleeders, but not all the time.
I put the proportioning valve in because every article I read said to even with drum/drum. I will try backing off the proportioning valve like Uncle Bob suggested.
thanks again for the input.

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Originally posted by 47coupelg
both fronts stay locked up until I crack the bleeders, but not all the time.

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Your master cylinder may already have check valves in it (I'm not familiar with that particular unit), and your adding another in each circuit could be a problem.