Technical guidance on dash valves, trip pressures

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Updated Feb 3, 2023
Red and yellow dash control valves for tractor protection and parking brakes.

[This article was originally published in 2019 by Trucks, Parts, Service. It has been updated to include more timely information.]

Understanding regulations for dash valve trip pressures are important to keep fleet and owner-operator customers compliant and safe. Below are vital tips that focus on red and yellow dash control valves for tractor protection and parking brakes.

“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,” says Bendix's Brian Screeton, technical training supervisor. “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.” (See diagram at end of the story.)

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, Bendix says. 

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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 will typically trip between 20 and 40 psi.

When it comes to the red button — the trailer supply valve that controls the tractor protection control valve — there is an FMCSA regulation covering trip pressure. Bendix says Title 49, Section 393.43, which addresses breakaway and emergency braking, establishes an “explicit requirement that the tractor protection valve or similar device operate when the air pressure — both primary and secondary reservoirs — on the towing vehicle is between [20 and 45 psi].”

The regulation is intended to activate the tractor protection for safety reasons if there’s a failure in the compressor or air reservoirs. It’s not intended to cover downstream failures like a failed trailer supply coupling, Bendix says.

“When the reservoirs deplete, the driver gets a low-pressure warning before the button pops and the spring brakes apply,” Screeton says.

“There’s also a bit of confusion created by that use of the term ‘breakaway,’” Screeton says. “Despite the wording, the ‘breakaway’ of the supply coupling is the most common means used to test the delivery function of the trailer supply valve and the tractor protection feature. Although the red button may trip at the specified pressure interval, it’s more common for it to trip instantaneously at 100-130 psi to protect the tractor service braking system.”

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To check the trip pressure of the red tractor control protection valve:

  • Install an accurate pressure test gauge in the secondary service reservoir
  • With both the red and yellow buttons pushed in, charge the system to air governor cutout
  • Turn off the engine
  • Disconnect the service or control line (blue) to the trailer, leaving the supply or emergency line (red) coupled to the trailer or suitable sealed dummy coupling
  • Take care to restrain the service coupling and direct the flow safely away, while making and holding a full-service brake application using the foot valve

As the service brake application is vented through the open control glad hand, the service system pressure will drop until the trailer supply valve — again, the red one — trips, shutting off the leak at the tractor protection valve created by the open control coupling. Record the pressure in the service reservoirs at this point and disconnect the trailer supply coupling to verify that the red coupling has been vented, thereby activating the trailer emergency feature, Bendix says.

“The CVSA tractor protection system check includes both the pop of the trailer supply valve — shutting off the trailer supply coupling flow — and the function of the tractor protection valve — shutting off the trailer control coupling flow,” Screeton says. “As noted, the tractor protection system is the one that is tested during roadside inspections and requires compliance. If the valve fails to close before 20 psi, it’s an out-of-service violation.”

It’s also a sign the valve will need to be repaired or replaced. Causes of such a failure can include wear and tear or air system contamination. Technicians should refer to the manufacturer’s Service Data Sheets for details and, where necessary, use OE-level replacement components.

Screeton adds, “We’re working continuously with partners across the industry to develop best practices for testing and maintaining air brake systems, and there may be different procedures used by some inspection agencies.”

Bendix dash valve configurations

An enlarged image of the dash valve configuration can be seen here.

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