GEAR 3T6576 - Caterpillar

Illustration 1 g00763283
View from the Bottom of the Operator Compartment (1) Service brake control valve
Illustration 2 g00774895
Service Brake Control Valve (2) Spring (3) Spring (4) Exhaust valve seat (5) Supply port (tractor) (6) Piston assembly (7) Exhaust port (scraper) (8) Inlet valve seat (9) Valve spring (10) Valve body (11) Valve (12) Exhaust diaphragm (13) Inlet port (scraper) (14) Exhaust port (tractor) (15) Plunger (X) No. 1 circuit (Y) No. 2 circuitThe service brake is composed of two circuits. The No. 1 circuit (X) is the portion between the spring seat which contacts the plunger and the relay piston. The No. 2 circuit (Y) is between the relay piston and the exhaust cavity.No. 1 circuit (X) of the valve is similar in operation to a standard single-circuit air brake valve. Under normal operating conditions, No. 2 circuit (Y) is similar in operation to a relay valve.Both No. 1 circuit (X) and No. 2 circuit (Y) of the brake valve uses a common exhaust that is protected by an exhaust diaphragm (12) .Application for No. 1 Circuit
When the brake pedal is depressed, plunger (15) exerts force on springs (2) and (3) and piston assembly (6). Piston assembly (6), which contains exhaust valve seat (4), closes valve (11). As valve (11) closes, supply port (5) is moved off the seat for supply port (5). The air for No. 1 circuit (X) flows outward through exhaust port (14) .Application for No. 2 Circuit
When supply port (5) is moved off the seat for supply port (5), air is permitted to pass through the bleed passage. The air then enters the relay piston cavity. The air pressure moves piston assembly (6), which contains exhaust valve seat (8). This closes valve (11). As valve (11) closes, supply port (13) is moved off the seat for supply port (13). The air for No. 2 circuit (Y) flows outward through exhaust port (7). Due to the small volume of air that is required to move the relay piston, action of No. 2 circuit (Y) of the valve is almost simultaneous with No. 1 circuit (X) .