How Trailer Brake Valves Work

A trailer brake valve is a device that controls the air supply to the tractor and the parking and emergency brakes of a trailer. It is typically a red eight-sided knob that supplies the trailer with air and shuts off when air pressure drops to 20 to 45 psi.

It protects and isolates the service reservoir from the emergency reservoir, which prevents automatic spring-brake application even when the service reservoir is lost (after a breakaway). This is an important safety feature that can help keep the trailer from slipping off the truck.

The valve is connected to the supply and service lines between the vehicle and the trailer via metal couplers called gladhands. These couplers must be kept clean and in good condition. The couplers can leak if not maintained properly and should be replaced as soon as they become clogged with dirt or grime.

An auxiliary brake system 12 is equipped with a hand-actuated auxiliary brake valve 34, which controls the pneumatic control circuit 37 that actuates the auxiliary brakes associated with the rear axle of the pulling vehicle. The auxiliary brake valve 34 is connected to a compressed air reservoir 36 by a reservoir line 35. The auxiliary brake valve 34 also connects to a control connection 43 of the module 13 that is jointly associated with the auxiliary brake system 12.

A relay valve disposed in the module 14 constitutes a component of a parallel redundancy in the compressed air brake system 10. It has two control chambers each of which is respectively connected to the control circuits of the service brake systems 11, and the auxiliary brake system 12, for the purpose of a pressure reduction or increase. The corresponding control pressures are conveyed from the control circuits of the service brake systems 11 and 12 to the control chamber 58 of the relay valve 14 in parallel.

The control electronics 45 modulate the electrical brake pressure request signal of the electrical control circuit 25 in accordance with a signal course and the signal level at the control input 59 of the relay valve 14. For a pressure increase, the control valve 14 switches its inlet valve 48 into an open position. For a pressure reduction, the control valve 14 switches its outlet valve 47 into an open position.

It is a component of the dual-circuit, dual-line, externally powered compressed air brake system 10 on the motor vehicle that can be operated for trailer operation. The compressed air brake system 10 has two control circuits that are separate from each other, of which one is connected to the front axle and one is connected to the rear axle.

These control circuits are each connected to a double-check valve that is linked to both the primary and secondary circuits of the compressor of the service brake system. The double-check valve only takes air from the highest pressure circuit, which prevents loss of air if the primary or secondary circuit fails.