Somerville, Indiana | jsmithsfarm - 12/9/2018 05:11
I have a Mack r model tandem grain truck. I started having problems with the back axle brakes locking up last fall. Put a new valve under the brake pedal and a new valve above the axle. Still didn’t fix it. Everyone said the shoes were wore down allowing the brakes to cam over. This fall I put new shoes on and it’s doing it again. It only does it if you get on brakes hard. When it does it air leaks out the bottom of the valve under brake pedal. If you stomp it a few times it finally releases. I would appreciate any suggestions. Thanks
I had trouble with an R model with Mack anchor lock system...The pancakes are Mack specific on old models and can lock up on a hard engaugement...It eventually got better...Never engage the parking brake (anchor lock for lack of better word) and press the brake pedal at the same time...It compounds the pressure on the shoes and tends to lockup..
Many will talk about caming over, but that seldom happens if it comes out of it...It can be the brake pedal valve or another valve in the system, or corrosion in the brake shoe assembly...If all are locking up it tends to be in the air system failing...Alcohol in the air system is your friend when it comes to getting rusty trucks road worthy again...An easy place to add is a plug on the air tank..
Edit: I think the ratchet in the pancakes at the cam shafts are rusty and locking up...I put light oil in the lines above the pancakes, seemed to help.
They are Mack's idea of a parking brakes, later on changed to the standard spring brakes...Expensive to change over the the spring brakes as they are larger, have to change cam shaft as well...Doubt that Mack parking brake chamber assembly can be sourced anymore...Hardy Brake used to rebuild them many years ago..
Parting thought, try not to push hard on the pedal valve...Seems like we had to push on the pedal to set the parking brake on the old R models...Three air lines going to the pancakes are a way to identify the old Mack system...Two air lines identify the trouble free spring brake chamber..
Edited by dave morgan 12/9/2018 16:40
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Mack truck brakes locking up
