If you own a heavy truck, you may have heard of the relay valve. This air-operated valve controls the brakes at the rear of heavy trucks, semi-trailers, and tractor-trailer combinations. The main purpose of this valve is to control the brake pressure and release when the truck is slowed down, and can be found on a variety of vehicles, including semi-trucks, commercial vehicles, and even bicycles.

The lower part of the valve acts as a standard service brake relay station. The bobtail proportioning valve reduces the normal service brake application pressure when the tractor is not towing a trailer. The BP-R1 has a control port connected to the Trailer Supply valve delivery. This valve also signals the bobtail's operation.

This replacement for the BP-R1 bobtail proportionation relay valve is made by Bendix*. This valve combines two different valve types into a single housing. The lower body acts as a standard service relay valve while the upper cover serves as a proportioning valve. This unit is lab-tested, carries the same warranty coverage, and is available at a fraction of the original price.

BP-R2 proportioning relay valve
The BP-R2 proportioning relay valve works in the braking system when the nett input voltage is less than the limiting pressure. In addition, the BP-R2 proportioning relay valve can also be used as a boost control device. In this way, the valve releases the braking pressure when the braking pressure reaches the required level. However, if the nett input voltage is greater than the limiting pressure, the valve will operate at a lower power level than is necessary.

The BP-R2 proportioning relay valve works by regulating the amount of electricity that flows through the proportioning circuit. The voltage is made positive during wheel acceleration and negative during deceleration. When the switch contact Sl is open, current flows to the capacitor Ci. When the switch contact Sl closes, the relay R is not energized. The switch contact S1 remains open during wheel deceleration.

The BP-R2 proportioning relay valve is a vehicle braking control device that includes first and second transducers responsive to axle load and antilock brake pressure. The first transducer produces an electrical signal indicative of each of these. A comparator device E responds to this signal and produces an output signal when the input signals have a relationship of IP-IA+ID. The currents IP, IA and ID are proportional to the voltages VP, VA, and VD, respectively. In addition, the current IB is proportional to a fixed bias voltage VB. This electrical signal activates the antilock modulating solenoid valve means.

BP-R3 proportioning relay valve
A bobtail proportioning relay valve is a combination of two individual valves in one housing. The lower portion functions as a standard service-brake relay valve, while the upper portion reduces the pressure of the service-brake application when not towing a trailer. This valve allows truck owners to have more control over their vehicle when not towing a trailer, as the pedal needs to be pushed further in order to apply enough air to stop the truck.