2
FIGURE 2 -
TYPICAL R-12P
™
PILOT RELAY VALVE SYSTEM SCHEMATIC
DELIVERY
SPRING
BRAKE
VALVE
SUPPLY
SERVICE RELAY
VALVE
QUICK
RELEASE
VALVE
VENTED
CUT-OUT
COCK
SINGLE
CHECK VALVE
VENTED
CUT-OUT
COCK
RESERVOIR
CONTROL
BRAKE CHAMBER
SLACK ADJUSTER
SUPPLY
CONTROL
R-12P
™
VALVE
OPERATION
CHARGING
When the air brake system is charging, the relay pis ton’s
exhaust seat is against the inlet/exhaust valve due to
the force of the reserve spring. Reservoir air enters the
R-12P
™
valve supply port and becomes active upon the
inlet/exhaust valve. At the same time, reservoir air passes
through the R-12P
™
valve body channel to the load piston.
Supply air builds simultaneously and equally underneath the
inlet/exhaust valve and above the load piston. While supply
pressure continues to hold the inlet/exhaust valve against
TRAILER BRAKE SYSTEM WITH R-12P
™
PILOT RELAY VALVE
its seat, it also moves the load piston into contact with the
relay piston. However, even though force is increasing on
the relay piston, it does not move the inlet/exhaust valve.
The effective area of the inlet/exhaust valve is larger than
the effec tive area of the load piston. Therefore, load piston
force cannot overcome inlet/exhaust valve force. So supply
air builds to governor cut-out and the inlet/exhaust valve
remains closed. This is the position of the R-12P
™
valve
in normal run mode, as shown in Figure 3.
-- -- -- -- -- ADDITIONAL PLUMBING FOR TOWING TRAILER
