Brake pedal goes totally to floor!

['70 C/10 SS500]

Just recently I started having problems with my brakes. While driving, eveything seams normal, the pedal has a good feel, the brakes are responsive and firm and really no problems at all. However, when sitting at a red light for a while, the pedal will slowly drop eventually all the way to the floor and the truck will start to roll forward. I have to quickly pump the pedal and then I instantly have a firm pedal and brakes again. I have manual brakes, NOTHING power. Today, I installed Russel speed bleeders and bled all 4 brakes till the fluid was perfectly clear and obviously no bubbles at all. The bleeders worked great, but the job was still a PITA because the bleeders worked so good and FAST that the master cylinder needed filling every couple of minutes. I was convinced that the thorough bleeding would solve the problem but unfortunately it didn't, the exact same conditions as before. I inspected all the lines and didn't see any leaks. I really am at my wits end here on this one. Anybody have any ideas?

[JimKshortstep4x4]

From what you are describing your master cylinder is leaking internally and needs replacement.

Jim

[GMCPaul]

I gotta agree with JimKshortstep4x4 your master cylinder seal is leaking and working towards complete failure. Swap it out or rebuild and you'll be good to go after a good brake bleeding.
GMCPaul

[Tx Firefighter]

about 20 bucks for a rebuilt unit at most parts stores.

['70 C/10 SS500]

Yeah, I kinda figured it was probably the master cylinder at fault. One question though, can a newer style M.C. be put on our trucks, the kind that has a plastic reservoir and screw on cap? I normally HATE most things plastic, but I have to admit that in this case a clear reservoir and easy on off cap makes brake work and fluid filling A LOT easier.

['70 C/10 SS500]

Well, to somewhat answer my own question about modern master cylinders being able to be used on our trucks, take a look at this pic of the Rad Rides truck. Knowing the budget and skill of this builder, this doesn't really mean much because he could probably make a brake system from a B-747 work, but at least it's possible. Anybody know how hard this would be to make work? Also, that M.C. looks like it might be off of a 4th gen F-Body, anyone know what kind of M.C. that is?

[elk hunter]

NEVER USE A REBUILT MASTER CYLINDER!!!!!!!!!!! your life and the lives of others count on your braking. b-smart i just replaced mine and a NEW master cost me 28 bucks wholesale. it might cost 35 bucks retail. always buy new.

[chickenwing]

PHHHFFT. You got just as good of chance of a "new" unit failing as a re-built one. Those re-manufacturing companies can make an as good as new unit. They'd get the pants sued off of em if they made shoddy breaking equipment. I don't have a problem rebuilding my own either.

[elk hunter]

lets make the bore size of a master cylinder that is designed to perform at a certain size larger. maybe we'll get all of the pitting out maybe we won't. its not like the cheap skate consumer will spend the 10 bucks to do the job right. all i'm saying is why buy something that has all ready worn out once when the price of a new master is less than going out to dinner. I have been in this buisiness for a long time and will never use a rebuilt master cylinder on any one of my customers cars. Ask a rep they will tell you the same thing.

[Tx Firefighter]

The amount of oversize due to machining during rebuild is negligable.

I have done this professionally for a long time also, and I can't remember ever having any issues with a rebuilt unit. I have a background in aircraft, so I'm probably unusually cautious about saftey matters, and even at that, I'm comfortable putting one on a customer's car, or even my wife's car.