To provide a brake application to the trailer unit only, a HAND VALVE (21) and a TWO-WAY CHECK VALVE (22) are piped into the system.

The purpose of the hand valve is to allow the driver to control independently the amount of application air to be directed to the trailer brakes when a trailer brake application is being made. It also provides a method of applying the trailer brakes when coupling the trailer to the tractor.

The purpose of a two-way check valve is to allow control of the trailer brake by use of the hand or foot valve. This valve EXHAUST will permit air to flow from the source which is supplying the MAIN RESERVOIR AIR PORT higher application pressures.

HAND OPERATED VALVES

For trucks that are intended to pull trailers, the hand operated valve is added to the air brake system to operate the trailer brakes. This valve allows the driver to apply the trailer brakes independently of the tractor. The amount of application air pressure delivered depends on the amount the valve is opened by the driver. Some valves must be closed by the driver; others will close automatically from any open position. Valves may be of a type that will remain in the full open position only, or they may be equipped with a locking device that will hold them in the desired position.

NOTE: The hand valve is NOT to be used for parking, as air may bleed off if the engine is stopped or the hand valve moves to the released position.

 

A TWO-WAY CHECK VALVE

This valve allows air to be directed to one delivery pipe from either of two sources. A two-way check valve allows the source applying the higher pressure to shift the shuttle so that the higher pressure will be directed to the delivery or ‘service line’. This valve is used between the foot operated valve and the hand operated valve, for the purpose of independently controlling the trailer brakes.

The driver has applied the brakes by using the foot valve. Application air is directed to the brake chambers of the tractor AND to the trailer brakes through a two-way check valve (22). The shuttle has shifted to the low pressure side, closing off any air flow toward the hand valve side. The hand valve (21) is in the closed position and equal pressure is being applied to the brake chambers of the tractor and the brake chambers of the trailer.

In this diagram, with the foot valve released, and the hand valve (21) opened, application air is directed from the hand valve through the two-way check valve (22), to the brake chambers. The two-way check valve in this application has shifted to the low pressure side, closing off any air flow toward the foot valve side.

The amount of application pressure through the hand valve depends on the amount that the valve is opened by the driver. But it cannot exceed main reservoir pressure.

Any time a trailer brake application is made by use of the hand valve, the driver may depress the foot valve treadle. If the foot valve application is of a higher pressure than that of the hand valve, the two-way check valve will shift to the lower pressure side, allowing the higher pressure to be directed to the tractor and trailer brakes.

If, during a foot valve application, the driver makes a higher pressure hand valve application, the two-way check valve will allow the higher hand valve application to be directed to the trailer brakes.

Regardless of whether the trailer brakes are applied independently by means of the hand valve, or together with the tractor brakes by the use of the foot valve, the maximum application pressure can only be the same as, or slightly less than, MAIN RESERVOIR PRESSURE.