BENDIX R-12P PILOT RELAY VALVE Manual - Page 5

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If, during the light brake application, the driver modulates

the treadle slowly, and not enough to open the R-12P

valve, the desired control pressure will still pass on to the

next valve, maintaining zero differential.

EQUALIZING (SLOW) EXHAUST (SEE FIGURE 8)

As described earlier, the result of any service brake

application with an R-12P

above and below the relay piston. If the brake valve is

modulated slowly upon release so that pressure beneath

the piston is not great enough to open the exhaust, delivery

air will pass through the equalizing oriﬁ ce and out the

control port to be exhausted.

REDUCTION OF SUPPLY PRESSURE

Figure 9 shows the R-12P

With no supply air, the reserve spring is strong enough to

hold the relay piston on the exhaust seat, but it is not strong

enough to overcome the force of the inlet/exhaust valve

spring. So both inlet and exhaust valves are closed.

Service Brake Applications with Reduced Supply

Pressure

Rapid Application - Use of Supply Line Single Check

Valve

If a rapid service brake application is made, control air moves

the relay piston and opens the inlet valve; but there is no

supply air to deliver. Control air then travels into the supply

port. The single check valve in the R-12P

port (shown in Figure 2) prevents this air from escaping to

atmosphere through the open or damaged supply source.

Control air also passes through the equalizing oriﬁ ce and

out the delivery port, assuring no reduction or loss of control

signal to subsequent vehicles.

Equalizing (Slow) Application

If an equalizing (slow) service brake application is made

with reduced supply air pressure, control air passes

through the equalizing oriﬁ ce and out the delivery port to

subsequent vehicles in the train.

Control air cannot escape to atmosphere because the

reserve spring keeps the exhaust closed.

™

valve is air pressure balance

™

valve with no supply pressure.

™

valve supply

™

inlet

LOAD PISTON

RELAY PISTON

SUPPLY PORT

INLET/EXHAUST

VALVE

FIGURE 9 - REDUCTION OF SUPPLY PRESSURE

WARNING! PLEASE READ AND FOLLOW

THESE INSTRUCTIONS TO AVOID PERSONAL

INJURY OR DEATH:

When working on or around a vehicle, the following

general precautions should be observed at all times.

1. Park the vehicle on a level surface, apply the

parking brakes, and always block the wheels.

Always wear safety glasses.

2. Stop the engine and remove ignition key when

working under or around the vehicle. When

working in the engine compartment, the engine

should be shut off and the ignition key should be

removed. Where circumstances require that the

engine be in operation, EXTREME CAUTION should

be used to prevent personal injury resulting from

contact with moving, rotating, leaking, heated or

electrically charged components.

3. Do not attempt to install, remove, disassemble

or assemble a component until you have read

and thoroughly understand the recommended

procedures. Use only the proper tools and observe

all precautions pertaining to use of those tools.

4. If the work is being performed on the vehicle's

air brake system, or any auxiliary pressurized air

systems, make certain to drain the air pressure

from all reservoirs before beginning ANY work

on the vehicle. If the vehicle is equipped with an

AD-IS

air dryer system or a dryer reservoir module,

be sure to drain the purge reservoir.

RESERVE SPRING

SEALS EXHAUST

CONTROL

PORT

DELIVERY PORT