Leak Test Fails on a Straight Truck Headed for Kansas City
You are doing a pre-trip on a 26,000-lb GVWR straight truck with a single air system and a 26-foot box. You charge the system to governor cut-out at 122 psi, shut off the engine, chock the wheels, and release the parking brake. Over exactly one minute the gauge falls from 120 to 114 psi. You then press and hold the service brake pedal fully and time another minute: the gauge falls from 118 to 111 psi. With the engine running you hear a faint hiss from the right rear area near the service chamber but cannot see a leak. The truck is scheduled to leave in 20 minutes with a produce load that must reach Kansas City by 6 a.m. The shipper has already loaded the trailer and the dock is needed for the next unit.
1. Apply the leak-rate standards to both tests. Pass or fail, and by how much?
Read the worked answer
For a single, non-combination vehicle, the static leak standard is no more than 3 psi per minute with the parking brake released and service brakes not applied; with the service brake fully applied and held, no more than 4 psi per minute. Released test: 120 to 114 is 6 psi in one minute, twice the limit. Applied test: 118 to 111 is 7 psi in one minute, nearly double the 4 psi limit. Both fail. The applied test failing by more than the released test is a clue: applying the brake adds pressure to the service side, so a leak that grows when the pedal is down often points to a service chamber, diaphragm, hose, or fitting on the service side rather than a supply-side leak.
2. How do you locate the leak, and what does the hiss near the right rear chamber tell you?
Read the worked answer
With the system charged and the engine off, apply soapy water to the suspected area: service chamber clamp ring, diaphragm area, pushrod seal, hose and fittings, relay valve, and the spring brake chamber. A hiss at a rear service chamber that worsens when the pedal is held usually means a ruptured service diaphragm or a leaking hose or fitting. That matters because on a spring-brake chamber the same housing contains the parking spring; a service-side diaphragm failure can be a precursor to a parking-brake problem, and on some designs a failed diaphragm lets air escape into the spring cavity. Do not open a spring brake chamber. It contains a compressed spring that can kill you. Replace the chamber or have a qualified technician do it.
3. What is your decision about the 20-minute departure, and what do you say to dispatch?
Read the worked answer
Do not depart. Both leak tests exceed the standard, and the vehicle is out of service until the leak is repaired and the tests pass: 3 psi per minute or less released, 4 psi per minute or less applied, for a single vehicle. The reasoning is that a 6-7 psi per minute leak is small at idle but compounds. Over a 30-minute drive with brake use, the compressor must constantly make up lost air, and if the compressor is already cycling at its limit, pressure can fall to the warning and spring-brake thresholds at the worst moment. Tell dispatch the numbers: 6 psi released and 7 applied, where the standard is 3 and 4. Red-tag the truck and offer the load to another unit or a later departure. A late produce load is cheaper than a brake failure on I-70.