It stops only when you turn the key off, then it unsticks, but the moment you turn the key back on it continues to run.

I agree that it appears to be a contactor assembly issue.
1. What was the condition of the tips before you replaced them ? Pitted?
2. What is the conditon of the battery - quite often if the battery is not up to full voltage this can cause excessive amp draw thru the contactor and tips resulting in sticking tips due to excessive arcing.

i think that your problem is in the contactor tips sticking together i have had this before on a different type of truck if you have changed the tips it could be worth changing the whole contactor assy for a slightly larger unit as the initial current draw on a hydraulic motor is high so if the contactor is on its limit the tips can stick intermitantly good luck

Sorry, I did not fully read your initial thread. If you've replaced the switches and contact tips, It has to be a problem with either a contactor or switch sticking- Or- either a control wire for the switches or cable to the motor is shorting out. Is it possible that the motor is loose and when it is turned on it moves a causes a short?

we have also replaced the control valve switches and all contacts.

model #tw30b
serial #tw235-211-3755

If it is a model that uses contactors, it is most likely a sticking control valve switch. Once you actuate the lever that is making the pump motor stay on you will see that the lever is attached to the control valve. There is a switch either near the top or the bottom of the control valve plunger. The switch is either out of adjustment or needs replacing.
The pump contacts could also be sticking. That would be caused by burned tips or by running the truck on a dead battery.

What Model# and Serial# is the Clark?