Source: Bendix
Elyria, Ohio – Call them the questions that keep “popping” up over the years: what are the regulations for dash valve trip pressures, and how can fleets and owner-operators stay compliant and safe? This installment of the Bendix Tech Tips Series will answer those questions with a focus on the familiar red and yellow dash control valves for tractor protection and parking brakes.
Regulated or Not?
“We periodically get questions about straight trucks, tractors, and buses being cited for dash control valves not automatically ‘tripping’ to the exhaust position at a predetermined system pressure,” said Brian Screeton, technical training supervisor at Bendix. “And while it’s not something we’re asked about every day, the confusion goes back a long way. The good news is, it’s easily cleared up with a quick review of what these two dash valves do, and how to test them.”
The parking brake control valve – the one with the yellow button – has no Federal Motor Carrier Safety Administration (FMCSA) regulations specifying the pressure at which it must automatically trip to apply the vehicle parking brakes. Parking brake “pop pressure” isn’t addressed in Federal Motor Carrier Safety Regulations (FMCSR) for in-use vehicles, the Federal Motor Vehicle Safety Standard (FMVSS) for newly manufactured vehicles, or the out-of-service criteria checked during inspections such as roadside checks by the Commercial Vehicle Safety Alliance (CVSA); although FMVSS 121 does require a single control for the park valves on a tractor-trailer and all the vehicles in a combination train. That said, the FMCSA Commercial Driver’s License Manual includes a note in the air brake section that says the parking brake valve should close, or “pop out,” at approximately 40 psi – a specification that isn’t supported by regulation, and which can cause some confusion.
To check the trip pressure of the yellow parking brake control valve:
- Install an accurate pressure test gauge in the secondary service reservoir
- Release the parking brake by pushing the yellow button
- Charge the system to air governor cutout
- Turn off the engine
- Open the manual drain valve (petcock) on the primary service reservoir to completely empty the tank
- Open the secondary reservoir’s manual drain valve and use the test gauge to create a bleed rate of 20-50 psi per minute
- Watch the test gauge and note the pressure at which the yellow valve pops out
The parking brake control valves used by most vehicle manufacturers – including the Bendix® PP-DC®, Bendix® PP-1®, and Bendix® MV-3® – will typically trip between 20 and 40 psi.
