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Air Brakes · Air Brake System Components

The first tank the air compressor fills has a safety relief valve built into it. What is this valve's purpose?

1.To vent air and shield the tank and the system beyond it if pressure climbs dangerously highCorrect answer
2.To warn the driver with an audible alarm whenever tank pressure drops too low for safe braking
3.To automatically drain accumulated water and compressor oil out of the tank on a timed cycle
4.To reduce pressure delivered to the front brakes whenever the road surface becomes slippery

Explanation

The safety relief valve pops open to vent air if pressure in that tank climbs too high, typically around 150 psi, which shields the tank and everything downstream of it from damage; a valve that actually releases air is a sign something else in the system has malfunctioned and needs a mechanic's attention. 49 CFR 383.111(b)(1) requires CDL applicants to know general air brake system nomenclature, the required-knowledge area this valve falls under; the specific 150 psi setting is the manual's typical figure, not a number stated in federal regulation. Warning the driver about low pressure describes the separate low air pressure warning signal, covered in the next Air Brakes topic. Automatically draining water and oil describes an automatic tank drain valve, a different component entirely. Reducing pressure to the front brakes describes the largely obsolete front brake limiting valve, not the safety valve.

Source: 49 CFR 383.111(b)(1)

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