Slidehammer it was.
The shaft was almost welded to the valve with carbon so took some getting out.
All's well now, valve cleaned and back in with a new seal because I broke the old one.

Big thanks again for pointing me in right direction.

Thanks for speedy replies.
The barrel was rebored before I bought it unfortunately but I'm pretty sure its just catching on carbon build up.
Its not enough to impede the piston but you can feel it touch if you move the piston in the bore. I'm hoping a thorough clean of the valve will be enough ...

Hi all. Reborn CRM owner here. I've actually owned mine about 12 years but due to life getting in the way its been laid up the last three. I was a member on here but it was so long ago I cant remember my username or password so have kept it simple and re-registered.
Any way I recently decided to do ...