"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.