Smith Airgun Repair
Authorized Crosman and Benjamin Repair Station
6012 Bayfield Parkway
found this over on the crosman forum. now if you have taken it all apart but cant get the valve out,? is that what your saying. to remove the valve, it has to be pushed forward but if its stuck maybe some wd40 will loosen the valve that may be gunked up around it.
now the headless screw, well there is a tube plug behind the valve, the headless screw you mention is called a thrust pin which is pressed into the tube plug, which is threaded on the top side which the breech screw , screws to. you need to use a small drift pin and knock it out then the tube plug should slide out the back side which will give you better access to the back side of the valve. using a dowel , push the valve forward.
check the cr/benji airgun gate, there are some manual there which will show you what I just wrote. HTH