A while back I made a post about the air brakes on the company bucket truck & I tried the foot valve as another member mentioned but it has what seems to be a bypass of air in it also & the brakes won't maintain pressure after popping the park brake off or release the brakes


There are no leaks in the system

It's a F750 with a 8.2 GM Diesel

 

So you, put your foot on the brake, release the spring brakes and the service brakes don't hold?

 

I attempt to release the parking brakes & it immediately seems to bypass inside the foot valve & blows air out of the valve under the hood

 

Does your air pressure build up as normal? Do you have the receiver drier on the truck? Are all the ports correctly attached, the threadle valve shouldn't leak any air, when you release the parking brake your charging the chambers, if you have a tear in one of those diaphrams it will force air back up to the foot valve and cause the air to come out of the open port, but the chambers have holes in them so you would hear the leak at the wheel. I would make sure the foot valve is in correctly, they are labeled and should come with spare plugs, to close off the none used ports.

 

I'll get my helper to try to release them & I'll listen for a leak It bulds pressure without any problems

 

Check your brake cans, I suspect one of them is bad. One of the signs of a blown diaphragm is a release of air at the foot treadle when applying the service brakes.


David

 

This ^^^^^

 

Sounds like a bad brake can bleeding air back into the service/emer valve on the dash

 

How do I tell what type of brake chambers I have?

Thanks for all the replies & help!

 

Take pics and post them here.

 

DO NOT TAKE THE CHAMBER APART. The spring inside can kill you or take off fingers.