The L-75 tractor brakes are piped similar to the T-75 system. A low pressure governor (31), a dump valve (32), a hand valve (21), a two-way check valve (22) and a relay type tractor protection valve (33), have been added. In this system, when air pressure rises to approximately 60 P.S.I., the low pressure governor opens, allowing air pressure to close the emergency dump valve. When maximum air pressure is reached, the governor unloads the compressor.

To fill the trailer tanks, the driver must open the trailer supply valve (24) until the trailer supply line gauge (34) shows over 50 P.S.I. The trailer supply valve will then remain open on its own, keeping the trailer tanks at the same pressure as the tractor tanks.

When a brake application is made with the foot valve, air flows through a two-way check valve (22) to the control port on the tractor protection relay valve (33), which relays air pressure from the supply line to the service line. The service pressure line relays air from the trailer tanks through the relay emergency valves (25) to the trailer brake chambers.

When the brakes are released, air pressure in the service line is exhausted through the exhaust port in the tractor protection relay valve. Air pressure in the trailer brake chambers is exhausted through a large exhaust port in the relay emergency valves. This action, both in application and release, is simultaneous on tractor and trailer.

To apply the trailer brakes only, or for a higher brake application on the trailer than on the tractor, the driver applies hand valve (21), air from the tanks flows through the hand valve into a double-check valve (22), to the tractor protection relay valve (33). Brakes on the trailer are then applied and released as described above.

If necessary, the trailer brakes can be ‘dynamited’ by closing the trailer supply valve. This shuts off the air supply from the tanks and simultaneously exhausts the supply line into the atmosphere. Loss of air pressure in the supply line causes the trailer’s relay emergency valves to dump full reservoir pressure into the brake chambers.

The trailer system will provide a rapid application and release of the trailer brakes as each axle has its own air tank and relay emergency valve.

If a brake chamber or air line ruptures on one axle, the other axle continues to have braking ability for emergency stops.

The L-75 tractor system has the same air loss safety features as the T-75.