VCCA Home Forums TECHNICAL - GENERAL Trucks and Vans 46 1/2 brakes

Have any of you guys ever had brake systems that "pump-up" as you drive them? My '46 1/2 ton has everything new except the drums and brake lines. The more I drive it the more the brake system pumps up until finally the brakes drag. I have to bleed off the excess brake line (trapped) pressure then the brakes work well until the sytem pumps up again. I suspected the new brake shoe return springs were weak, so I replaced the return springs with another set of new ones. That didn't do anything, so I rebuilt the NOS master cylinder thinking the rubber parts in the MC maybe had gotten bad over the years - again no difference. Everything on this system is new except for the drums and steel brake lines. I mean the wheel cylinders, brake line hoses, brake hardware, wheel bearings and seals, king pins etc.
I adjusted the shoes so I feel them just grip the hub, push the brake pedal to make sure the shoes are centered, them back off the shoes until the wheel spins freely. I did this adj. with the vehicle cool - not warmed up.

When I restored the vehicle I flushed the lines before I installed them - since they were good I didn't see any reason to replace them.

I'm kind of stumped - Any ideas?
Thanks guys!
BTW, have fun in CO, wish I were there!

Jim
Have you checked the rod or rods between the brake pedal and master cylinder. Sometimes the replacement cylinders are not machined to the same depth as the original (been caught with genuine unit) causing the piston to block the return port in the cylinder.
Try placing a flat washer between the master cylinder and its mounting points.
Tony

I've had brake hoses act a one-way valves. I'll push on the brake and it doesn't release.
If steele line is krimped, it will do the same.
Start looking where you have to bleed.

I had the same problem with my 37 and tonyw has the fix mentioned right on his post. I worked with that problem for at least a year and finally one day just took the lid off of the master cylinder and watched it. The piston did just that - blocked the return port in the master cylinder. I made the adjustment and have never had the problem again. I suggest you check this point out and good luck!

Joe