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Had similar issues with frozen brakes, only the fronts were solid rust and the rears sort of worked. the shoes were WAY out of adustment and the M/C leaked like mad, it was not a fun start to the trip home (I decided it would be cheaper to pay for a tow before the accidednt rather than after).


The common failure part I saw (was even stuck on my NOS part) is the relay valve. It's about a 1/4" bore and doesn't take much to stick it. Without that the M/C pressure wont open the vent and allow the vac press to work the booster, a total disasembly and clean is essential. Luckily you can still get rebuild kits for these things, or there's a company in the NorthEast that my local Napa was gonna send my old one to before I lucked into a NOS part on E-pay. And as a last resort, brand new ones are available from bendix, they are still used on forklifts.

Good luck!