D3B transmission question

I pulled the engine for a rebuild and took out the 3F/3R transmission too. This machine had nearly 10,000 hours on it. It barely crawled in 3rd, was weak in 2nd, and wouldn't spin the tracks in 1st when I tried to move a stump. When I tore down the transmission today I was really surprised to find what looked like brand new discs and plates. All five clutch stack-ups measured right in the middle of the NEW tolerance according to the CAT powertrain SPECIFICATIONS manual. The teflon piston rings on four of the pistons are loose in the bores with NO resistance when I place them in. If those loose rings were causing the clutches to slip, wouldn't the plates be all burnt up? Could the torque converter be my slippage problem instead? Is the TQ in a D3B known as a weak link? Any thoughts would help.

Pictures attached.

Check the orifice on the bottom of the pressure control valve—that's the top part of the valve body. It's plastic and can break or get plugged, which cuts off the flow to primary pressure. That'll definitely make the transmission feel gutless.

Is the PCM orifice located inside the valve body on the top of the transmission?

Which one are you asking about?

12 and 19 orifice and seal

Yeah, they're underneath that cover with all the bolts on it.

You have to take out all the 5/16 bolts, remove the top plate, then flip the next section over so it's on its side facing the clutch packs. Those plastic orifice tops have a habit of coming loose - they used to be pretty cheap to replace.