I. What’s Relay Valve? 

From Wikipedia: a relay valve is an air-operated valve typically used in air brake systems to remotely control the brake at the rear of a heavy truck or semi-trailer in a tractor-trailer combination. Relay valve are necessary in heavy trucks in order to speed-up rear brake application and release, since air takes longer to travel to the rear of the vehicle than the front of the vehicle, where the front service brakes, foot valve, parking-control valve, and the trailer-supply valve(if applicable) were located.

In short, relay valve can shorten brake reaction time and pressure establishing time.

Structure of relay valve

looks complicated? See following photo

How does relay valve work?

1. Compressed intake air is waiting at ①. it can’t move unless something press the core. 

2. When we brake, control air incoming will push diaphragm though ④ port and the small air hole. 

3. The moved diaphragm press the core of valve, there will be an gap between intake port ① and whole valve chamber, the then the intake air can flow though ㉑ ㉒ to bake chamber. 

4. When brake action released, core return to original place since the spring pressure, gap disappear, intake air flow cut off, the air return from brake chamber will be released to atmosphere though the gap between core and diaphragm and ③ exhaust port. 

This is the full working process of the relay valve. 。

I. Assembling place of relay valve 

As we said above, relay valve can shorten brake reaction time and pressure establishing time. So the fixing place is at the middle of two rear axle. You will find it easily.

Hope this article is helpful for you. Anything unclear, contact me.