It sounds like a problem with the inching valve or the inching master cylinder. If you take of the floor board and depress the left hand pedal you should see a lever move on the top of transmission. That is the inching valve. Either it is working intermittantly or not returning properly. There is a spring and seal kit available for the inching valve. It sounds like an easy fix (for someone that is familiar with forklift trucks). You might want to call a dealer and have them repair it before you go ripping things apart.

THANKS! This truck ***. If you step on the brake on a incline it justs keeps rolling backward and if you are going uphill, such as loading a trailer, halfway up it starts to slip. If you put it in neutral for a few seconds then back in gear it works.

It seems to me that the model should be C20B and I think that you are missing a digit in the serial #. It should probably be 291-813 which puts it at a 1966.
The hydraulic filler cap which is also a breather should be on the right hand side of the engine as you are sitting on the seat and the master cylinder should be located on the right hand side under the frame near the drive tire.