When I depress the brake pedal I hear an air sound in the cab as if the brakes are losing some type of air pressure. This truck does not have air brakes and prior to my purchase I was told that the master cylinder was replaced. Any ideas??

ricalbuq

 

It sounds like you have a ruptured diaphragm in the brake booster.

 

what exactly does that mean? Where is the brake booster. Forgive the ignorance.


ricalbuq

 

It means you have a hole in the diaphragm inside the brake booster.


Boosters play a very important part in the flow of force though the braking system. The illustration shows a vacuum booster mounted just before the master cylinder. The connection to the brake pedal linkage is shown sticking out the rear of the booster.

The booster makes it possible for people with all degrees of leg strength to activate the brakes effectively. The booster adds a vacuum or hydraulic boost to the force produced by the driver's leg and the leverage of the brake pedal. All these forces combined push on the master cylinder. With the boost working, a much lower force is required to operate the brakes.

 

I was told that pumping the brake pedal with the motor off the pedal should not bleed off, it it does the booster could be bad.

My is making a heck of a whooshing sound of air under the dash, only when engine running and pumping the brakes.

Sounds like a booster getting ready to let go? What are some braking warning signs that it is about to quit working?

I just replaced the master cylinder a few months ago, then over the weekend the a rear wheel cyclinder let go. So I repalced both wheel cylinders and shoes, bleed the system and now I have the ABS light. I went to the ABS valve and bleed it a couple times no air, soild fuild. The light was never on until after the master cylinder was replaced but before the wheel cylinders let go.

Ideas? Thoughts?

Thansk Mike

 

Check the wiring on the master cylinder to make sure it is making a good tight connection. I had a nagging ABS light and figured out that it was the connection at the master cylinder when I replaced the master cylinder and found the wires loose.