For stubborn valve stems, I have two steps: (1) Apply PB Blaster to the base of the stem where it threads into the piston top. Let it sit. Let it sit for several days if you're so inclined. PB Blaster can pretty much free up anything that can be freed up. (2) Instead of trying to unscrew the stem from the piston, think "backwards" or "upside down". Unscrew the piston from the stem. The way to do this is to first remove the valve cap from the top of the stem (again, use PB Blaster if this is a problem). Then go to your vise and insert the rubber jaw pads (You DO have a vise and rubber jaw pads, right?). Then firmly clamp the valve stem in the padded jaws of the vise with the piston body pointing up above the vise. Then CAREFULLY grasp the piston (using a cloth or rubber fabric) and unscrew the piston from the stem. This works in cases where you'll never be able to unscrew the stem from the piston by holding things in your hands.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)