The Stovebolt Site Forums The Shop Area General Truck Talk Problems after installing new master cyclinder w/ booster

After I replaced the single reservoir master cylinder with a dual reservoir/booster kit I have had difficulty brakes remaining adjusted. I have 4 drums without discs so no proportioning value was needed. After adjusting for proper pedal height I experience a gradual decrease in pedal (tops out) until it can actually lock out the brakes. At that time if I readjust the push rod length so that the brakes are not locked out the pedal later will nearly go to the floor after the truck sits. I suspected that heat was a problem and shielded the lines from heat from the exhaust pipe. This didn't seem to help either. Any thoughts on this will be greatly appreciated.

Did you install residual pressure valves? Should have a 10psi valve on each front and rear for drum brakes.

Heat should not be an issue.

Did you bench bleed the master cylinder and get all the air out?

Do the residual pressure valves help when the pressure seems to be building up? Seems like the pressure keeps going higher not lower??

james50, this site:
Typical Brake System
says you need a metering valve for the front and the residual valves.
Are you sure you have a master cylinder for drum brakes?

Make sure you have at least an inch of free travel on the pedal before you feel the master.Sounds to me that the master is set to tight.



Pete

I have to say the first point under #1 on the link MBM is wrong. It is disturbing to see a company selling brakes when they don't understand how they work.

I don't quite understand the nature of the problem you are having. When brakes are "locked out", what does that mean? Also, I don't think you will ever find a factory setup that does not include residual valves so I would make sure that you have them in the system, either in the master or separate.

locked up.. as in the pedal is up with no pedal play.

New questions:

1. Does it matter which reservoir goes to the front wheels? The reservoir closest to the power booster or farthest from the booster? Currently the line going to the rear is farthest from the booster.

2. When bleeding brakes from rear to front, I notice that the pedal goes to the floor when the rear wheel cylinder is opened and only 3/4 down when the front wheel cylinders are opened during bleeding. Is that normal?

It sounds to me like your brake shoes are adjusted too tight.

If your brakes seem to work fine first thing in the morning, or when the truck is cold, but then as you drive it you get less and less pedal travel and the brakes start to drag until they lock up, your brakes are probably too tight at the wheels. When this happens you will feel a lot of heat at the wheels. Your brakes are heating up causing the brake fluid to expand.

Adjusting the master when the brakes are hot wont help because when the truck cools, your pedal travel will increase until the pedal hits the floor.

I hope this helps, but I would still do some more research on residual valves.