Applying brakes (Fig. 12.15(a)) Depressing the foot treadle applies a force through the graduating springs to the pistons, causing the exhaust hollow stem seats for both pistons to close the inlet/ exhaust valves. With further depression of the foot pedal, the piston simultaneously unseats the inlet/exhaust valves and compressed air from the reservoirs passes through the upper and lower valves to the front and rear brake actuators respectively (or to the tractor and trailer brake actuators respectively).

Balancing (Fig. 12.15(a and b)) With the compressed air passing to the brake actuator chambers, air pressure is built up beneath the upper and lower pistons. Eventually the upthrust created by this air pressure equals the downward spring force; the pistons and valve carrier lift and the inlet valves close, thus interrupting the compressed air supply to the brake actuators. At the same time, the exhaust valves remain closed. The valves are then in a balanced condition with equal force above and

Fig. 12.15 (a and b) Dual concentric foot valve

below the upper piston and with equal air pressure being held in both halves of the brake line circuits.

Pushing the treadle down still further applies an additional force on top of the graduating spring. There will be a corresponding increase in the air pressure delivered and a new point of balance will be reached.

Removing some of the effort on the foot treadle reduces the force on top of the graduating spring. The pistons and valve carrier will then lift due to the air pressure and piston return springs. When this occurs the inlet valves remain closed and the exhaust valves open to exhausting air pressure from the brake actuators until a state of balance is obtained at lower pressure.

Releasing brakes (Fig. 12.15(b)) Removing the driver's force from the treadle allows the upper and lower piston and the valve carrier to rise to the highest position. This initially causes the inlet/ exhaust valves to close their inlet seats, but with further upward movement of the pistons and valve assembly both exhaust valves open. Air from both brake circuits will therefore quickly escape to the atmosphere thus fully releasing the brakes.

12.3.11 Dual delta series foot control valve

Purpose The delta series of dual foot valves provide the braking system with two entirely separate foot controlled air valve circuits but which operate simultaneously with each other. Thus, if one half of the dual foot valve unit should develop a fault then the balance beam movement will automatically ensure that the other half of the twin valve unit continues to function.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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