how can I release an old (maybe 25years) Drayton TRV?

I have removed the top of the TRV and pulled the pin up. The pin came out.is this supposed to happen? The pin is housed in a plastic housing inside a brass cap which is held in place by an internal circlip. Can I remove the circlip without causing a leak? If so can I get access to the valve plunger to free it off? I haven't tried banging it yet as one website discouraged this.