B737 Park Brake System Description
The Park Brake Valve is located in the return (system B) line of the NORMAL Anti-skid valves.
It`s purpose is to block return fluid from leaking through the normal anti-skid valves back to return (to system B reservoir).
By blocking the return line of the normal anti-skid valves the park brake valve ensures that the accumulator when fully charged(3000psi) will hold the aircraft for eight hours with-out the use of chocks.Parking for longer periods requires the outboard wheels of the MLG to be chocked.
The park brake valve will have some affect if the brakes are parked using system A.System A pressure over time will bleed down through the Alternate anti-skid valves as there is no park brake shut off valve preventing the leakage of pressure to return to system A reservoir.
Pressure leakage will not occur through the Normal or Alternate Brake Metering Valve Assy`s.
When the A system pressure within the brakes drops below the accumulator charge pressure, the accumulator isolation valve opens allowing accumulator pressure to hold the brakes in the parked position.As the accumulator uses and is in the NORMAL (system B) hydraulic system the park brake valve stops the accumulator pressure from returning throught he normal anti-skid valves just as it would if the brakes had been parked normally.
Regards DDG