Trailer Brake Releases Slowly on One Side
A 2019 53 ft dry van with tandem axles and spring brakes comes in after a 40 km highway run. The driver reports the trailer pulling right during the last few stops and the right rear wheels feeling hot. Infrared readings are 190 °C on the right rear drum and 55 °C on the left. With the parking brake released, the right rear chamber pushrod takes about 9 seconds to retract; the left retracts in under 2 seconds. At 90 psi the right pushrod stroke is 44 mm (1.75 in); the chamber is a Type 30 with a 51 mm (2 in) adjustment limit. There is no audible leak at the chamber, but a gauge teed into the chamber inlet falls from 90 psi to 20 psi almost immediately when the brakes are released, then creeps to 0 psi over 8 seconds. A gauge at the relay valve delivery port drops to 0 psi in under 1 second. The trailer has ABS and the ABS lamp is off.
1. Identify the most likely cause of the slow release and explain what the two gauge readings prove. Why is replacing the brake chamber not justified by the pushrod stroke alone?
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The relay valve delivery port drops to 0 psi in under 1 second, so the relay valve is exhausting normally. The chamber inlet gauge falls quickly to 20 psi and then bleeds down slowly, which shows a restriction between the relay valve and the chamber. The most likely cause is a collapsed or internally degraded inlet hose, or a fitting with a blocked or swollen inner liner. The restriction lets pressure enter but slows the exhaust path, so the spring takes several seconds to retract the pushrod.
The 44 mm pushrod stroke is below the 51 mm limit, so the chamber is not overstroked and the reading does not condemn it. A long stroke would indicate adjustment or chamber problems, not a slow release. The temperature difference confirms the right brake is dragging, but the pressure test localizes the fault upstream of the chamber. Replacing the chamber would leave the restricted hose in place and the drag would continue. The correct sequence is to confirm free flow through the hose, replace the hose and fittings if restricted, then check the chamber for heat damage and inspect the drum and linings for cracking or glazing.
2. Outline the repair and post-repair verification. What would you do if the right drum still runs hotter than the left after the hose is replaced?
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Replace the restricted hose with the specified hose and fittings, route it without kinks or tight bends, and confirm the chamber pushrod retracts in roughly the same time as the left side. Re-adjust the slack adjuster if the stroke is outside the planned stroke range after the repair. Make several controlled stops from about 30 km/h and compare drum temperatures side to side; a small difference is normal, but a repeated 50 °C or greater spread means further diagnosis.
If the right drum still overheats, do not assume the new hose solved everything. Check the chamber for a weak or broken return spring, verify the slack adjuster is not seized, inspect the foundation brake for a dragging shoe, a seized camshaft bushing, a broken spring, or a drum that is out of round. Also verify the relay valve is not delivering residual pressure and that the ABS modulator is not holding pressure on that wheel. Use a pressure gauge at the chamber during release and a dial indicator on the drum. Replace only the confirmed faulty component, then repeat the temperature and release-time checks.
3. Is this trailer out of service under CVSA criteria? Explain how the temperature and release-time findings affect your decision and what documentation you would complete.
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A brake that drags enough to overheat can be an out-of-service condition even when the pushrod stroke is within limits, because the stroke check does not test release performance. The 190 °C drum and the 9-second release time show the brake is not releasing properly, so the unit should not be released until the fault is corrected and the wheel end is checked for heat damage.
Document the initial drum temperatures, release times, gauge readings at the relay valve and chamber, the replacement hose, and the post-repair temperatures. Note that the pushrod stroke was within the adjustment limit. If the drum, lining, or wheel seal shows heat damage, record that and the corrective action. A trailer that has been overheated also warrants a wheel-end inspection for grease condition and seal leakage before it returns to service.