HELP!! Stuck valve in Haley QB .308 conversion

Need some help… I have aquired a Jack Haley QB conversion and it has been a problem since day one.. Now the valve is stuck open and I have tryed high pressure blast from my tank several time working the bolt and firing while adding air, removing the hammer and bouncing the air tube on carpeted floor to no avail.. Help me get this working again please.. Since Jack is ill I won’t bother him.. How do I remove the valve or can send it to someone to have it fixed.. This is my first big bore and so far a exercise in frustration.. Only airgun I have now…
Thanks,
Ray