Master cylinder or booster possibly gping bad?

[GM-power]

My brakes have been working normal until this morning. Crank the truck this morning and let it warm up( it was sitting outside in the cold) start driving and the brake pedal os super soft. Sometimes ail braking the pedal would lose "pressure" and go all the way to the floor. Bled the brakes this afternoon and there wasn't any air in the lines. Bleeding them didn't do anything. Don't have any leaks anywhere. My rear pads are pretty worn but I didn't think that would make the brakes do that. Anybody got any thoughts? This happened out of the blue so im kind of clueless.
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[bigwheel15]

Sounds like the master cylinder to me, a bad booster will make the pedal stiff.
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[THE "10" SILVERADO]

Bad MC. Get a new one NOT a rebuilt one. I got a rebuilt one and its JUNK. Going to get a new one myself.
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[gchemist]

Test it!!
http://www.73-87.com/7387garage/chas...on/booster.htm

[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.

[GM-power]

Well I put a brand new MC on today. It feels like it has consistent pressure but that it has nothing to brake with. My rear pads are super worn down. So I may replace the rear brakes. The feeling I get now is hard to describe. Going down my driveway which is a incline the pedal is definetly high and holds good. Would replacing my rear pads have a effect on my braking. That's about all I can think of. And trust me when I say my pads are worn cause they are super worn.
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[mooseknuckles]

change the rear pads/shoes, make sure adjusters not sticking...

[tucsonjwt]

I installed the new master cylinder cover gasket and no more leaking but not much improvement in brakes. I will pull the cylinder away from the booster and see if it is leaking there.

[INSIDIOUS '86]

Simple diagnosis on brakes
The rear gives you your Pedal feel and the front gives most of your stopping power.
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[GM-power]

Well I went for a longer test drive today. Definatley going to do a rear brake job, drums are in really bad shape to. Thanks y'all for all the help!
Tuscon, if it was leaking between the booster I think you would see it.
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