Help I have a bostitch

Question

Help
I have a bostitch N80 coil nailer that leaks from the main part that is held together by the 4 allen bolts, I have checked the tightness and when i re tighten them the leak stops for a while then reappears soon after.
I think the operation of the gun loosens the allen bolts.
I am unsure as whether or not their should be a silicon gasket smeared before the two parts of the gun are reattached?
Or should it just be a bare metal contact between the two?
Thanks
Neil