Hi All, looking for a bit of help with a 2nd hand F28WW nailer I own.  I have watched the diagnosing nailer problems video on youtube, which brought me here. 

The problem I have been getting is after firing the nailer, the hammer gets stuck forward and begins to hiss air from the body.  This problem happens intermittently (usually fairly commonly when first using, and decreases with more frequent use.  I am a weekend warrior so it doesn't get alot of use).  Typically, to get the gun functioning again, I disconnect the air source, remove nails, push the hammer back with an allan wrench (these are the steps needed ~50% of the time).  The hammer is straight and is never difficult to push back into position.  Other times I have needed some love taps and I have felt a pop happen in the gun which appears to always solve the problem.  ~ 1-2 years ago I replaced the O rings with a rebuild kit.  None of them appear flat or worn at this point.  Looking for direction.  Wondering about the trigger valve or others.  Hoping this is easier to diagnose for others with experience.  Any help is appreciated.