Re: drum brake lockup issue...

"Nate Nagel" wrote: '55 Studebaker Commander

Rear brakes lock up under somewhat hard braking.

Stock 4-wheel drum brakes Stock under-floor single reservoir master cylinder. Has "anti-creep" electrical line lock which activates only when the engine idles to kept the car from creeping forward. Wires to the anti-creep have been cut but brake lines have never been replaced; the valve is still there. I have replaced all shoes, all wheel cylinders, cleaned everything, replaced all hoses and master cylinder. Pass side front drum: The adjuster is backed all the way off and it drags just a little more than I prefer. _____________________________________________

BRAKE SHOE DRAG:

Shoes should not drag at all. When released, the pivot end of the shoes should stay near but not touching the drum, while the brake springs pull the moving shoe ends inward and squeeze the fluid out of the wheel cylinder, back into the master cylinder and reservoir. A relief valve stops the shoes from further retracting when the proper residual line pressure is reached. If a shoe drags it may be caused by:

a.) Parking brake left on. b.) Wrong shoe - curve does not match drum. c.) Weak shoe return springs. d.) Jammed cylinder piston. e.) Master cylinder pushrod not fully released. f.) Leading/trailing shoes interchanged. g.) Front and Rear shoes interchanged. h.) Creep-valve not bypassed. i.) New shoes not broken in (10 hard stops). j.) Adjuster too tight. l.) Defective master cyl relief valve. m.) Front/rear Master cyl lines interchanged. n.) About 50 other things I can't remember.

BRAKE SHOE LOCKUP:

Take a drum off, have someone push on the brake a little (not far enough to blow the pistons out of the wheel cylinder). Observe whether the springs quickly and smoothly retract the shoes. The shoes should stop after retracting, but should not retract all the way to the stop post. If it retracts all the way, the residual line pressure relief valve is defective

There may be nothing wrong: Rear tires can skid during hard braking because the larger front brakes do most of the work and the load on the rear axle gets lighter when stopping. The rear brakes may temporarily grab, but release when pressure is released.

Brake grabbing may also result from oil on the shoes.

If it is the wrong shoe, the pivot end may be too far from the drum, causing the self-energizing feature of drum shoes to jam the shoe into the drum.

Front brake shoes or pistons should be larger than those of rear brake shoes to lessen rear tire skidding. If not, there may be proportional valving in or near the master cylinder to reduce the pressure applied to the rear shoes.

Good luck.

Rodan.

Reply to
Rodan
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I would look at the brake shoes again to determine that all four shoes are identical. If you notice that two are thicker and have a different friction material length from the other two then you have "self-energizing" brakes. The thicker shoe is called a "lead" shoe. It of course is the front shoe whilst the shorter, thinner shoe is the trailing.

Shoes that are too thick will not allow the shoes to "energize", or pivot on their posts mechanically "wedge" against the drum. You say you have one that is dragging even with the adjuster fully retracted, and this should be looked at closely.

Re-check for leading shoes, re-bleed, re-adjust. If you replaced the master cylinder you might want to look at the actuator rod length; if it is too long the master cylinder may not be able to function correctly. That is if it is indeed the correct master cylinder.

DieInterim

Reply to
DieInterim

Can you find a brake shop--possibly an older, established one--that has a machine to "true-arc" the shoes? In older days, we had drums mic'd and the machine set up to match, and true the shoes so as to fit 'perfectly' the circumference of the drums. Sure made for an all-around fit between shoes/drums. The slightest high spot will cause a pumping and friction action that heats and swells the shoes, leading to.....you guessed it, a lockup! HTH, s

Reply to
sdlomi2

I wish... I actually thought about that, since none of the shoes have perfectly worn to the drums yet. But no, I don't - if anyone knows of one, preferably not too far a drive from Annapolis, MD - I'd be all ears.

nate

Reply to
Nate Nagel

Reply to
Shep

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