MIDLAND-ROSS RELAY VALVE
The Midland Ross air relay valve is a piston type, air
operated valve used to deliver and exhaust air being
supplied to brake chambers. It is also designed to
provide a quick means for applying and releasing the
service brakes. The valve is remotely controlled by air
pressure from the foot valve.
When the brake valve is applied the relay valve permits
direct air pressure, supplied from the air reservoir, to
pass and apply the air chambers. When the brakes are
released the relay valve also permits the air chamber
pressure to exhaust out the valve into the atmosphere.
The air pressure from brake valve is also influenced by
the action of modulator valve in the anti-lock control.
Fig. 1 Midland-Ross Relay Valve
The relay valve is mounted on the frame close to the
brake chambers it serves. Fig. 2 illustrates the relay
valve when the service brakes are fully released.
When the brake valve is applied air pressure enters the
relay valve at the control port and into cavity "A." Air
pressure above piston moves it down and seats the
exhaust valve against modulation tube closing off
exhaust passages. Further downward movement of
piston forces modulation tube down and opens inlet
valve by moving away from valve seat of body.
Reservoir air pressure enters cavity "B" through reservoir
port and then passes through delivery ports (not shown)
and to service brake chambers.
Fig. 2 Cross Section of Midland-Ross Relay Valve
Exhaust Valve Assy.
Inlet Valve Assy.
Exhaust Valve Body
Exhaust Valve Spring
Air is blocked from exhausting through modulation tube
center. As air pressure builds in cavity "B," it tends to
move piston upward. When the air pressure above and
below piston approaches a balanced condition, piston
moves upward until inlet valve closes against valve body
If air pressure coming into the control port is held
momentarily at braking condition, the relay valve attains
a "lap" position. Lap position implies that both inlet and
exhaust valves are seated simultaneously. When the
valve is in this "lap" position it is instantly responsive to
any change in pressure from the brake control valve to
increase or decrease vehicle braking. Air pressure
outputs of relay valve are directly proportional to the
signal air pressure at control port.
CTS-4079 - CHAPTER VI -PAGE 2