It could be dirt but also it could be a lack of pressure. Check that the machine has the correct Hydraulic pressure. Low hydraulic pressure will not allow the system to overcome the built in check valves.

Replaced with rebuilt control valve. No drift but sometimes it won't tilt forward. Dirt maybe???

tilt spool will be damaged internally ,only way forward is a new one. there is little flares on the spool that randomly break off and cause this problem

The tilt spool will have a check valve either internally fitted in the spool or externally in the pipe work. This check valve is to prevent the drift forward as the pressure is matched in the system to the pressure in the tilt rams. The lift spool has a similar single acting check valve to prevent the mast dropping as the pressure is matched when you go to lift. 3rd and 4th slices are not normally fitted with check valves and using either one of these slices to run the tilt could cause a problem and would not be the best way forward. I would recommend if parts are available replacing the check valves in the tilt circuit or replacing the tilt slice.

Thx Mrfixit ,I will try that and see if it helps

You know, I did have that problem with one of those Hysters. After overloading my brain, I noticed the valve had a 3rd spool that wasn't being used. I moved the tilt hoses over to it and connected the lever and bingo it worked perfect. So, I think that tilt spool has something special about it, maybe a tilt lock feature that was malfunctioning. Anyway, it had to be something wrong with the tilt valve spool, but the customer said he was happy the way I switched it and don't pull the valve out for repair.