View Full Version : Brake Anti lock and relay valves

Joe Camper
11-12-2016, 07:51 AM
I love it when I go through service records on a chassis and find a problem others had uncovered and were unable to crack.


This is an antilock valve. Most of u have a few of these. They r plumbed into the service air lines that supply the brake chambers.



This is a r12 brake relay valve. It is what controls when air is allowed to go to the antilock and chamber.

These valves like the brake tredle and the air dryer too have exhaust ports at the bottom. Another similarity is when they r leaking from the exhaust that quite frequently is not the valve itself that's bad.

Case in point, this antilock was leaking that way but the antilock was not bad. It was its associated r12 valve that the diaphragm went bad on allowing tank air to bleed backward out the exhaust on the antilock key off just sitting, draining the brake tanks.

Anytime u here the exhaust on the brake pedal doing this that too is almost always a relay leaking not the foot pedal.

Happy camping!!

Joe Camper
11-12-2016, 07:07 PM
Going further a brake relay serves the same purpose as an electrical relay only its newmatic.

When u step on the brake the air that is sent is not just going right to the hubs. Right off the dimension of the line would have to be huge and even then there would be fade and delay. Closer axles to the driver would react a quicker than axles further away.

The relay eliminates that.

What u have is every axle with a air tank and a relay purposely designed so the 3 components to be all right next to each other right at the axle. The Air thats coming from the brake pedal supplies the relay with just a signal that then opens a very short path for air thru very big hose from the brake tank through the relay and to the chamber (usually prob less than 3 or 4 feet total) and wala, no fade, no delay.

Hope that helps a bit to better understand service brakes and what a brake relay is and what its doing.