[tucsonjwt]

Odd that you should mention this. I just got my 83 C20 out of the equipment shop with a bad hoist cylinder after being there for 2 months - both start batteries at about 10% charge, so I've had it on my little charger for the past two days. But on the way home from the shop I had bad booster/cylinder like symptoms - very soft and little braking.

So, I took the cover off my 2 year old Raybestos master cylinder and found lots of rust on the top sealing edge of the cast master cylinder body, both surfaces of the rubber gasket, and the underside of the cylinder cover. I used wet sanding paper and a little of the residual brake fluid to clean off the top surface of the cylinder and the bottom surface of the cover, and got a good MIC (Made In China) gasket for $8 from NAPA.

I installed a Delco rebuilt booster last summer, so I hope I don't have a cylinder that leaked into the booster. The booster was definitely leaking around the cover gasket, so I am hoping that was the problem.

I will post results after a test drive once the batteries are back up to 100%.
Hard to believe I could have so much rust in AZ. My neighbor says moisture precipitates out of the brake fluid and condenses on the top master cylinder surface -IDK - I never saw this in the many old cars/trucks I have owned over the many years.