Vacuum braking system is a system which uses only the vacuum for the application of brakes.
WORKING OF VACUUM BRAKE SYSTEM
The brakes are always in released condition with vacuum unit the driver pushes the brake pedal. In this condition position of piston cap is in cap end of the brake cylinder and cam is twisted for compressing the spring which provides free rotation of drum.
When the driver pushes the brake pedal slowly then the flow control valves open slightly to the atmosphere. It loss of vacuum causes the brake to be applied due to spring force. When the flow control valve opens fully then alternatively the forward direction.
The direction of flow is changed and atmospheric air enter through the exhaust port of direction control valve to piston cap end. Due to pressure difference the piston moves backward with vacuum and spring force.
The movement of link rod attached with piston rod releases the cam to normal position which makes internal resistances from the brake shoes against drum.
When driver release the brake pedal then valve lever comes back to initial position. The direction of flow is again changed and atmospheric air enter through the exhaust port of direction control valve to piston rod end. Due to pressure difference the piston moves forward with vacuum.
The movement of link rod attached with piston rod twists the cam and compresses the spring which makes enough clearances between brake shoes and the drum. This release the brake.