Where is the relay valve typically located in an air brake system?
- In the supply reservoir
- Between the brake pedal and the foot valve
- Between the reservoir and the rear brake chambers ✓
- Between the primary and secondary reservoirs
Answer: C. Between the reservoir and the rear brake chambers
The relay valve is located between the reservoir and the rear brake chambers.
Why this is the answer
The handbook describes the relay valve as installed between the reservoir and the rear brake chambers, and it is usually mounted on a frame rail near the chambers it operates. Air reaches it straight from the primary or secondary reservoir through a large supply line, so a big volume is ready to move to the chambers quickly. A control signal from the foot valve tells it when to deliver that air, which shortens the lag time on both application and release.
Why the other answers are wrong
- A. In the supply reservoir — The reservoir stores the compressed air and feeds the relay valve through a large supply line; the valve itself sits downstream of it, not inside it.
- B. Between the brake pedal and the foot valve — The foot valve is the control that sends the signal to the relay valve, so the relay valve is not placed in the line between the pedal and the foot valve.
- D. Between the primary and secondary reservoirs — The passages say the relay valve is fed from the primary or secondary reservoir and mounted near the air chambers, not positioned between the two reservoirs.
Remember it
Picture the relay valve as a helper sitting close to the rear brake chambers, fed by a big line from the reservoir and triggered by a small signal from the foot valve.