Spring Brake Valve
The function of the spring brake valve is to supply a specific, limited holdoff pressure to the spring brakes, and in the event
of loss of service pressure, to modulate the spring brakes through the use of the dual brake valve.
The valve has four air connected ports and a diaphragm protected exhaust port. Each air connection port is labeled with
embossed letters to identify the ports.
Initial charge air, from primary and secondary reservoirs flow through the park control valve and enters the spring brake
valve supply port. Air entering the supply port flows past inlet and exhaust valve B to the under side of piston B and out the
delivery port of the spring brake valve to the emergency air connection at the spring brake actuator. Air flowing from the
primary reservoir only enters the reservoir port of the spring brake valve.
This air remains under piston A as system pressure builds. With primary reservoir pressure below approximately 55 psi
(379 kPa/3.79 bar), the spring above piston A forces it into contact with inlet and exhaust valve A causing the exhaust to
seal and the inlet to open.
With air system pressure above approximately 55 psi (379 kPa/3.80 bar) in the primary and secondary reservoirs, piston A
has moved against the force of the spring above it, allowing the inlet of valve A to close and opening the hollow exhaust
passage through piston A. When air pressure beneath piston B is approximately 95 psi (655 kPa/6.55 bar), piston B rises
slightly against the force of the springs above it, allowing the inlet of valve B to close. The exhaust through valve B
remains closed. The closing of the inlet portion of valve B traps approximately 95 psi (655 kPa/6.55 bar) in the hold-off
cavity of the spring brake actuators while allowing full air system pressure to build elsewhere.
When a service application is made by actuating the dual brake valve, air from the No. 2 delivery circuit is delivered from
the brake valve to the control port, and is stopped at the closed inlet of valve A. No movement of the internal components
of the spring brake valve takes place. Air from the No. 1 delivery circuit of the dual brake valve actuates the service
section of the spring brake actuators.
With both system No. 1 and No. 2 intact when the park control valve is placed in the park or exhaust position, the spring
brake valve supply of air pressure and the air pressure in the spring brake actuator cavities is exhausted. The single
check valve in the spring brake valve assists this exhaust of air pressure from the spring brake by allowing the air below
piston B to flow back out the open exhaust of the park control valve. When air pressure below piston B has dropped
sufficiently, piston B moves down opening the inlet of valve B thus providing an additional exhaust passage for air
exhausting through the spring brake valve from the spring brakes.