Leak Test and Pushrod Stroke on the Same Pre-Trip
Before departure you run the air brake tests on a tractor-trailer. With the system charged to 118 psi, engine off and brakes released, pressure falls to 113 psi after one minute - a 5 psi drop. With the brakes fully applied and held, that same 118 psi falls to 104 psi in one minute. You then measure pushrod stroke on the left rear drive-axle chamber: a clamp-type Type 30 chamber shows about 2 1/8 inches of stroke at full application. That wheel has an automatic slack adjuster, and its adjusting pawl looks freshly greased. The rest of the chambers look normal and the low-air warning checks out.
1. Interpret both leak readings and explain what the difference between them tells you about where the air is going.
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The two tests isolate different parts of the system.
- Static test (brakes released): checks the supply side - compressor, reservoirs, and everything that holds air regardless of whether you are braking. The limit for a combination vehicle is 4 psi per minute, so a 5 psi drop fails, though only slightly.
- Applied test (brakes held): adds the delivery side - treadle valve, service lines, relay valves, and brake chambers. The allowable rate is higher than the static limit, but still only a few psi per minute. A 14 psi drop is far outside it.
Reading them together: because the static test also failed, there is at least one leak that exists with the brakes released, plus a second leak that only appears under application. A chamber diaphragm or a service hose that weeps only when pressurized is the classic cause of that pattern.
Next step before condemning anything: soap-test every chamber, hose, and fitting on both the tractor and trailer with the brakes applied and held, and again with them released. Repair everything found, then repeat both tests - a leak test is only valid after the repair is verified.
2. The stroke is 2 1/8 inches against a 2-inch reference for that chamber. What could cause that, and why is simply shortening the pushrod the wrong move on an automatic slack adjuster?
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What long stroke can mean:
- The automatic slack adjuster has stopped adjusting (seized, damaged, or installed wrong).
- Worn linings or drums using up the available travel.
- Loose or worn foundation parts - a cracked chamber bracket, a wallowed clevis pin, a bent pushrod.
- The wrong size chamber installed.
Why not just turn the adjusting nut: a slack adjuster that is not maintaining stroke is broken. Winding the pushrod in restores the measurement temporarily and hides the failure, and on an automatic unit it can damage the internal adjustment mechanism. You would drive away with a brake that looks compliant on the gauge and is still not self-adjusting.
Decision: a pushrod stroke at or beyond the limit is a brake defect and the wheel is out of service until repaired - not 'adjusted and monitored.' Combined with a failed leak test on the same unit, this vehicle does not leave the yard. Repair, then re-measure stroke on every chamber and re-run both leak tests before dispatch.