I’ve never experienced this before. This morning temperatures in Ohio was in the mid 20’s when I jumped into the 34 pickup. Stepped on the brake and lost all brake pressure. This has a single pot master cylinder running 40’s brakes. I continued down the country road warming the engine and pumping the brakes. As the engine and brakes warmed up my brakes returned to normal to finish the 20 mile trip. My brake fluid is full. I’m not running pressure relief valve. Would a brake pressure relief help??
By "lost all brake pressure", do you mean the pedal went to the floor? If that's so, I think the cup in the master cylinder is old and very hard. I believe a master cylinder rebuild or replacement is in your future.
With no fluid loss I would suspect that the master cylinder seals shrunk and was byp***ing internally. Then when they warmed up they started to seal again ????
Yup, in years past when working on cars with single master cylinder they often would show signs of failure (pedal sinking) when the weather turned cold. Dual masters will too, but I experienced more with single cylinders. Worn / hard cylinder rubbers don't seal well when cold. Time for some new parts.
Ok thank you guys!! It’s was a NOS master installed a few years ago. The hardened rubbers in the cold has to be the problem. Thank you. On the way home my banjo growled a bit and I totally lost gearing to the rear axle. Had to tow it home. Ain’t old cars fun!!!
That’s where this pickup started originally. From there to Texas and then it was a partial basket case to Ohio. Time to pull the rear end, at least I’ll have plenty of room to pull the master cylinder. Thanks again!
I've had the same problem with my 48, usually bleeding the left front restores the brakes. I still haven't figured it out but then I'm not changing any parts either.
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