Frozen tail stock clamps HiT 18S

They hydraulic tailstock clamps seem to be frozen solid. The tail body moves OK, but it won't lock in position. The tail spindle slowly pushes it back until I get a limit alarm and the program stops. I have been putting a clamp in the way behind the tail body to keep it from moving, but I would like to get the pistons working. I can't turn the pistons using the big hex on top. I have used breaker bars and even an impact wrench.
I am afraid to apply much more force since I could open a hydraulics leak.
I have started looking for a tail stock from a scrap machine.
Any suggestions are appreciated.
Regards, Ray