Photo's would help people to help you. If you upload pictures to a remote host such as photobucket.com you can insert a link in your forum posts to the pictures.
The gun should cock without a trigger guard, but the trigger adjusting screw goes through the guard and without it the trigger will sit fully forward. That might be the problem.
A breech seal is easy to make from a piece of scrap leather, or you might find a suitable size 'o' ring to fit, or even cut a slice from a piece of plastic or rubber tubing.
The buttplate would be metal, not rubber.
I collect vintage Japanese air rifles & vintage Japanese pellets
Information sought about antique firearms with Japanese markings, do you have one ?