Operation and maintenance manual

Section 250-0070
Braking System - Treadle Valve
SM 2165 Rev 1 11-03
2
DESCRIPTION
Numbers in parentheses refer to Fig. 1.
Note: The treadle valve can be identified as item 6 in
Section 250-0000, BRAKING SYSTEM SCHEMATIC.
The tandem circuit modulating treadle valve is a
closed centre controller which controls the level of
hydraulic oil pressure applied to the front and rear
brakes and the maximum pressure available to these
circuits. It is operated by a foot pedal in the operators
cab and with the engine running, is automatically
applied by the brake manifold valve. Refer to
Section 250-0050, BRAKE MANIFOLD VALVE.
The tandem circuit modulating treadle valve consists
of an upper and lower spool, the lower spool being of a
smaller diameter. As the foot pedal is depressed the
upper spool is pushed down and the lower spool is
pilot applied by the pressure in the brake circuit of the
upper spool.
In the normal position, brake pedal assembly (1) and
upper and lower regulator spools (16 & 14) are in the
up position. In this condition the notched area near the
top of regulator spools (16 & 14) is exposed to the
tank cavity in valve body (9 & 10), the lower portion of
the notched area is exposed to the regulated pressure
outlet ports of the valve. This effects a direct link
between the tank port 'T' and the regulated outlet ports
'B1' and 'B2'. While upper and lower regulator spools
(16 & 14) are in this position the brakes are released.
A proximity switch (28), which requests the
transmission retarder, is fitted to the treadle valve. The
first 7
o
of pedal travel will activate the proximity switch
(28) and request the transmission retarder. A warning
light on the dash will also be illuminated by pressure
switch fitted at transmission. The normally open switch
closes at 0.3 bar (4.35 lbf/in²). Further depression of
the treadle valve then applies the brakes. Refer to
Section 120-0010, TRANSMISSION AND MOUNTING.
A pressure switch located at a tee in the 'B2' line
energizes (illuminates) the vehicles stop lights on
application of the treadle valve.
OPERATION
Numbers in parentheses refer to Fig. 1.
Normal Service Brake Application
Refer to Fig. 2. When the operator depresses pedal
assembly (1), piston (7) is moved down and pushes
on regulator spring (18) which in turn, pushes upper
regulator spool (16) down and pilot pressure pushes
lower regulator spool (14) down. As regulator spools
(16 & 14) move down, the metering notches move out
of the tank cavity and close outlet ports 'B1' and 'B2'
off to tank port 'T'. Regulator spools (16 & 14) continue
to move downward until the lower edge of the
SM - 2767
SM - 2768
Fig. 3 - Normal Service Brake ReleaseFig. 2 - Normal Service Brake Application
PX
P1
T1
P2
B1
B2
T2
6
7
TO FRONT
BRAKES
TO REAR
BRAKES
SUPPLY
FROM
BRAKE
MANIFOLD
SUPPLY
FROM
BRAKE
MANIFOLD
PX
P1
T1
P2
B1
B2
T2
6
7
FROM FRONT
BRAKES
FROM
REAR
BRAKES
SUPPLY
FROM
BRAKE
MANIFOLD
SUPPLY
FROM
BRAKE
MANIFOLD