At what pressure do air brake system safety valves typically open?
- 80 psi
- 100 psi
- 150 psi ✓
- 200 psi
Answer: C. 150 psi
Air brake system safety valves normally open at 150 psi.
Why this is the answer
The handbook says the supply tank and air dryer can have safety valves that protect the system from too much pressure. If the governor does not tell the compressor to cut out and pressure keeps rising, these valves open and let excess air escape. The handbook gives the normal opening point as 1035 kPa, which is 150 psi. It also says a safety valve that is venting means pressure is too high and needs immediate repair.
Why the other answers are wrong
- A. 80 psi — Eighty psi is used in other air-brake checks, such as low-pressure and build-up tests, not as the safety-valve opening point.
- B. 100 psi — One hundred psi appears in governor and build-up checks, but the handbook does not give it as the pressure where safety valves open.
- D. 200 psi — Two hundred psi is higher than the opening pressure the handbook states for safety valves.
Remember it
Think of the safety valve as the 150 psi backstop: if it vents, the governor failed to cut out the compressor.
Official handbook: Official MTO Air Brake Handbook, Air Supply Subsystem – Safety valve