The Stovebolt Site Forums The Parking Lot Big Bolts Brake problem on my 56 GMC 300 dually

Heres some backround on what I've done to my 56 GMC 300 dually brakes.I've adjusted up all the brake shoes then I bled the brakes to the hydrovac unit then out to the wheels, furthest one away first.
Here's what happens the pedal will pump up to a good pedal,then when left alone for 30 seconds or mor it'll go to the floor,and will pump back up again.When looking into the brake fluid reservoir when the pedal is left alone there is brake fluid returning to the reservoir and filling it up.It's almost seems like I havn't adjusted the brakes up and the brake springs are pushing fluid back to the reservoir.
Any ideas ?
Thanks
Tim

Hy bigtim44, did you do some work on your brakes, requiring bleeding? Did you bleed both ports on the hydrovac? It sounds like you have air trapped in the system, the other thing would be the brakes not adjusted up, but you said you have already done that.

Tim,

I'll be interested in seeing what people have to say.

Your symptoms are exactly what my 54/55 350 GMC COE did after I replaced all wheel cylinders and the master cylinder (and new brake shoes, adjusted properly). I bled the entire system 4 times (about 2 quarts of fluid) using a pressure-bleeder, all as described in the GMC maintenance manual.

I drove it about 15 miles to storage and I had to pump the pedal in anticipation of braking; the pedal got quite hard and braking was excellent. Nonetheless, after braking, the pedal went to the floor each time.

I had the hydrovac rebuilt but have not yet installed and tested it.

Tim

In my '53 pickup I had an issue where the brakes were hard and worked great, but if you stood on them you could slowly feel it going to the floor. I pulled the master and rebuilt it. There was a small hole worked into the side of the cup on the piston made by some small piece of debris, I almost missed it. Fluid would slowly return behind the piston when holding the brakes.

Jeff

I had the same problem and adjusting the shoes seemed to cure the problem. I had already replaced the brake hoses and rebuilt the master cylinder. One problem we run into was a small return hole in the master cylinder was choked off. There are two holes in the master cylinder. Good luck

Pictures???

Thanks guys ,so I'm not the only one who's had this problem!
I'll crank the adjusters up till the wheels don't move,and see if that makes a difference to the pedal.I'll try rebleeding the hydrovac aswell,then start changing parts.The whole system is original but in good condition with no seepage.
I'll keep ya's posted
Cheers
Tim
Theres some pics of the truck in question at the bottom of 1 and top of page 2 here: http://smg.photobucket.com/albums/v312/bigtim44/

When putting new brake shoes on you really need to turn the drums and arch the shoes, providing the drums don't go out of limits, if not what you'll have is just a portion of the shoe touching when you adj the brakes but when you apply the force is much greater and the shoes will give to fit the drum, when turning and arching you make the two surfaces more compatible.

Tim,

You really should replace EVERYTHING on the brake system. The steel and rubber lines will fail from the inside out. The old brake fluid wicks water and causes rust to form on the inside and the rubber lines will swell on the inside. Don't take chances with your truck...be safe. The materials to make it new are cheap and it's easy to do.

Mike B