Just picked up a 2500 Cummins and it will not shift into 3rd. I can lift off the throttle and it will up shift but sometimes takes awhile. Any ideas?

Your description is not very clear or complete. Does it get third when you tip out of the throttle all the time but not when you are on the throttle? Or never gets third? Does it have reverse all the time? Or no reverse? If you have reverse and get third on tip out - check the throttle kick down cable for freeness. There should be a return spring from the throttle lever to a bracket at the trans end. Check and make sure this cable goes wide open then returns all the way forward before doing anything else.

front clutch piston seals is the first thing that comes to mind.

or governor weights/sealing rings.

Same here. I remember some models we had updated front drum , piston and seals for that type of problem. But I dont remember which models and years it was as I would have to look it up but I do remember replacing them during the 90's on some for that problem when everything else was working correct. Reverse usually still worked ok since it runs with much higher pressure. Ron

I've done some that had a poor finish on the inside sealing surface of the high gear piston. This caused the piston to bind, especially when cold, The trans would shift into 2nd and then not shift into 3rd. There is a bulliten for this condition.
Doug.

The 500 had a poor seal material issue - I do not recall seeing an issue with the 518/618. That was in the late 90's on the 500 but may have started earlier. We went from a cheaper rubber material to Viton for the fix. The cheap material heat aged quickly and shrank.
A lot of the kits use a thicker seal design for this seal. There were reports of mismachiend clutch retainers where the seal groove was too deep. Maybe the thicker seal was made for those situations. As for binding pistons on the 727/518/618 - never saw that. The scratch marks usually seen are from contact but the only way you see a bound piston is not keeping the release springs evenly spaced around the piston.

And while we are guessing - how about a sticky 2-3 shift valve, 2-3 governor plug, worn reaction shaft seal rings, sticky throttle pressure valve, others?

don't make me get the brake fluid out....

Yep- brake fluid works too - for a while at least.

Shifts fine when cold, 1st, 2nd, 3rd, OD. When warm takes awhile and about 2500 rpms before it shifts to 3rd. Reverse is fine. If I lift off the throttle it usually go into 3rd. I will check the throttle cable. Yes, return spring is intact.

Even though the front clutch is also used in Reverse, a strong Reverse is no proof that the clutch is good because the line pressure increases dramatically in Reverse; so a weak clutch will still appear to be OK in Reverse.