The parts that are causes for air leaks from the exhaust are listed below from most common to least common:
1) o-ring on the inside diameter of the head valve piston
2) o-ring on the outside diameter of the head valve piston
3) cylinder seal
4) head valve piston
5) cylinder cap
6) cylinder sleeve -- check the area where the cylinder seal seats on the cylinder sleeve
Note: the o-rings and seals are included in all o-ring/rebuild kits, or the individual parts can be ordered separately. See Service Parts posting on this site.