CDL Air Brakes TestAir Brake System PartsMedium
A commercial vehicle is equipped with a dual air brake system. A driver notices that after fully charging the system to 125 psi, the primary air tank loses pressure at a rate of 4 psi per minute with the engine off and brakes released, while the secondary tank holds steady at 125 psi. What does this indicate?
- AA restriction in the air line leading to the primary air tank.
- BA leak in the primary air tank system.
- CA faulty air compressor governor causing over-pressurization of the secondary tank.
- DA normal pressure drop due to temperature changes.
Show answer & explanationAnswer & explanation
Correct answer: B. A leak in the primary air tank system.
A pressure drop of 4 psi per minute in one tank while the other holds steady indicates a leak specific to the primary air tank system (tank, lines, or components connected to it). Normal leakage for a combination vehicle (engine off, brakes released) should not exceed 3 psi per minute.
Why the other options are wrong
- A. A restriction would prevent the tank from filling properly, but not cause it to lose pressure once filled, especially if the secondary tank is full.
- C. The secondary tank is at 125 psi, which is the cut-out pressure, so it's not over-pressurized, and the problem is specific to the primary tank.
- D. While temperature can cause minor fluctuations, a consistent 4 psi/min drop in one tank is excessive and indicates a leak.
Dual Air System Leak Check
A procedure to test for air leaks in a dual air brake system, performed by observing pressure drop in each isolated air tank with the engine off and brakes released. Maximum allowable leakage rates are specified by regulations.
- Vehicle must be fully charged (125 psi).
- Engine off, parking brake on, brakes released (no foot pressure).
- Combination vehicle max leak rate: 3 psi/min; single vehicle max leak rate: 2 psi/min.
Memory trick: One Tank Drops, Other Holds? It's a Leak's Old Hold!