12H inching pedal sticking

Have a Cat 12H with approx. 11500 hrs. It has a sticking inching pedal. You have to put your foot under the inching pedal to get the spool to return to its "standard" position. The grader won't drive when the inching pedal is not fully released, as expected, as the inching pedal spool is 'dumping' all the pressure. I can remove the cable from the spool and the cable moves freely along with the pedal; it appears to be the spool sticking (I have been told that the cable has been replaced). You have to lift the inching pedal with your foot to return the spool all the way. So it is also when the cable has been removed and you manually move the spool, after pulling it and then releasing it, it looks like it returns to its normal position, however, you have to push it to get it to return to its true position. Thought it may be the wiper seal at the end of the housing, as it felt quite tight when trying to move the spool, so loosened up the seal but to no avail. The o-ring and seal located at the end of the housing have since been replaced. The spool isn't scored or marked in any way that would it to blame for the sticking. Springs look fine also. Should mention that I tried another inching pedal spool assemble from another control valve but to find the same symptoms. Has been an on-going job that I've been on and off with so will recheck my work next time I'm going to it. Worst case scenario, I have another valve slice to try to see if the valve housing is at fault. My questions would be: 1. What are the obvious and simple things I should recheck before getting carried away in my diagnosis? 2. Is it possible that there is some gallery blocked/restricted or even damaged in some way that could result in this symptom? Note: this particular machine has had dirty oil go through the trans before due to a collapsed filter but has since been stripped and CLEANED, especially the control valve, but hence my latter question above.