Sounds like the seal pack will have to be replaced, some times it can be rebuilt , but often it needs to be replaced.
When I have had to take one out of the valve..
I remove the valve from the tank, then remove the front control face from the valve body, then remove the clip that holds the plug at the rear of the valve body.
There is a socket that will just fit in the hole between the 4 little legs and the inside of the hole. Then you can use some thing to pound down on top to the socket, if you do not have a socket that will fit, then a screw driver handle that just fits inside those 4 legs and pound down. it is best if this is done on the corner of a bench or the like with the shaft of the piston sticking down..