Max,

CONTROL VALVE
The control valve is generally composed of relief valve, forward-reverse selector valve, inching mechanism and cold relief
valve.
The hydraulic oil is sucked out of the transmission case by the gear pump attached to the transmission housing. The oil
delivered out of the pump enters the relief valve of the control valve where the clutch oil pressure is regulated to about 199 PSI
(14 kg/cm-varies by the RPM 's.
The oil that flows out of the relief valve enters the torque converter. A cold relief valve is built in the control valve for
preventing abnormal rise of oil pressure in the oil converter due to increase of oil viscosity in colder temperatures.
The oil of regulated pressure flows through the inching mechanism and forward-reverse selector valve and then causes the
forward or reverse hydraulic clutch to operate.
The inching mechanism is provided to engage and disengage the hydraulic clutch without selective motion of the selector valve.
It works in conjunction with the brakes. This is the basic operation of the transmission. You may need to remove the control valve and check all the ports to make sure nothing is blocked. I have not been able to find pressure port checks. I also included the clutch operation.

HYDRAULIC CLUTCH ASSEMBLY
The forward clutch engages when the clutch oil is led to the side of clutch piston "F'' and forces the clutch piston "F", causing
the drive plate to be pressed between packing plate "F" and clutch piston "F". As a result, torque is transmitted to the forward
drive gear assembly.
For reverse drive, the clutch oil is led to the side of clutch piston "R", and torque is transmitted to the reverse drive gear
assembly, which reverses through the reverse gear.
A feature of this clutch is that pistons on both sides do not operate individually but they always work in a unit. When oil
pressure is applied to the forward side, the forward moves. The reverse piston also moves as pulled by the forward piston, and
spring on the reverse side is compressed. When the neutral position is selected, pistons are pulled backward by the force of the
spring on reverse side, and the forward clutch is disengaged. Opposite motion occurs when oil pressure is applied to reverse
Sheet 34: Transmission--Disassembly
side.
OPERATION
As the operation of this clutch is hydraulically controlled, neutral, forward, reverse, etc. are accomplished by lever control only.
FORWARD OR REVERSE
When the lever is set in forward (or reverse) position, the oil which was regulated to approximately 199 PSI (14 kg/cm2) as
stated in the previous paragraphs flows through the inching valve stem side and then flows by way of a notched groove in the
forward-reverse selector valve and clutch oil hole in the control valve body and to the respective holes in the clutch shaft and
pushes the forward (or reverse) clutch piston.
As a result, the forward (or reverse) clutch operates and the truck moves forward (or backward).
Hopefully this will help. If i find pressure checks will forward. Good Luck.