How about a sticking valve. I have seen the foward/reverse valves on the trasmission do wierd things. Maybe you could swap them and see if the problem follows them.
Maybe you could do the compressed air trick. Hook a air hose to the pressure test port, or better yet right to the shaft if you can access it, for forward and then reverse. Listen closely to the clutch packs to see if you can hear the forward one leaking.
Replaced piston seals and seal rings
Did you replace all the piston ring seals, the big one on the outside of the piston and the little ones that are on the shaft?
Are these steel rings that have locks the hook together where the gap is? I had this problem once with a Yale. Once it warmed up it slipped in forward and that shaft piston ring was worn enough that it had expanded all the way until the lock wouldn't let it open enough to make a good seal. Cold it would go ok but once warmed the thinner oil and leak-by past that shaft seal caused the clutch slippage.
Original pressure test showed 90 psi forward, 200psi reverse, which to me indicated bad piston seals. rebuilt transmission but never could find any thing obvious causing low pressure, thinking the problem was in the control valve possibly i installed trans and installed a different control valve. yesterday i rechecked pressures but i connected the gauges while the truck was cold, both clutch packs 200psi but as the truck warms up the forward pressure gradually drops to 90psi when up to operating temp. im thinking possibly a cracked piston or something. one of the local dealers rebuilt three of these transmissions for this customer about a year ago, replaced pumps, control valves and converters and never could get them to last more than a few months before they would start slipping again
I'm not familiar with the inching arrangement in this truck but Clark usually uses a piston cup and spring set up. If that's what you have it sounds like a broken spring that is not allowing the piston cup to return.
Did someone misdiagnose this problem and you had the transmission rebuilt because of it?
Does it happen with reverse also? I would agree with mrfixit, it sounds like an inching problem. Maybe the rod or cable are hanging up just enough to engage the inching slightly.
Something must be wrong with the inching piston.
Model is C25L Serial P232L 0494 9645. TA30 is the transmission. transmission has been rebuilt, converter replaced, oil cooler flushed and control valve replaced. Clutch pressure 180psi. Transmission works perfect as long as you dont use inching [email address removed]
Do you have the model and serial number of the truck and an email address to put in your profile?
I would start by testing the transmission(TA30) Pressures,especially the forward pack pressure.
Is TA30 the forklift model number? I never heard of that one.