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Why do large trucks use a relay valve for the rear service brakes?
- A.It replaces the need for a foot valve
- B.It filters oil from the compressor
- C.It speeds up brake application and release by letting a large volume of air reach the rear chambers quickly
- D.It charges the batteries on long trips
Correct answer
C. It speeds up brake application and release by letting a large volume of air reach the rear chambers quickly
Why
Air traveling all the way from the foot valve would cause noticeable lag at the rear axles. A relay valve uses a small signal from the foot valve to release a large local supply of air, cutting lag and making front and rear brakes apply together.
Manual topic: Air Brakes - Air Brake System Components · From the Air Brakes practice bank (AB-010).