small dump truck brake problem

[GT2]

Ive got a GMC 6000 dump truck that the brakes stopped working.
It has a hydrolic master cylinder on the fire wall and a vacuum booster under the cab.
The pedal goes to the floor, no leaks anywhere and the fluid level stays up.
I'm thinking master cylinder?
Any suggestions?
Thanks, Tim

 

[Bob Rooks]

Sounds like a master cylinder to me too - by-passing. Does the fluid bubble if you depress the brake pedal slowly? What year is the truck? How old is the brake fluid?

 

[GT2]

1978, cant see cylinder while pumping, I'm by myself.
Fluid about 3 years old.
Thanks

 

[Bob Rooks]

Make sure all of your vacuum line connections are good and tight.
Are there brake lines going to the booster?

 

[jaotguy]

to the floor and no leaks = master

rebuild or replace , bench prime and install ... then bleed brake system

 

[GT2]

Make sure all of your vacuum line connections are good and tight.
Are there brake lines going to the booster?

Yes. lines to booster.
Any chance the booster cylinder is bad?

 

[tcartwri]

might just be the pushrod between the pedal and the booster. not unheard of.

 

[Bob Rooks]

Yes. lines to booster.
Any chance the booster cylinder is bad?

Yep, that's the usual suspect, and the most expensive unfortunately.

 

[Ed of all trades]

I am told that some years of GM trucks had a problem with trapped air in the master cylinder. Knew one guy that drilled and taped a vent hole in his and vented it as he added fluid, He said it worked.

 

[GT2]

I got the old master cyl off and was able to see the problem, there was rust on the backside of the cylinder bore that was preventing the piston from returning all the way.
New master cylinder on and back to work!
Thanks for everyone's help.
Tim