impact fps loss

Sometimes the piece on the end of the valve stem (think Ernst calls it the plunger)comes loose and unwinds or unscrews over time. 
It should have a small grub screw to keep it fixed but they often aren't there. The valve stem can also unwind from the other end at the valve. Which is harder to see or check. If you look at the trouble shooting paper it has the distance the plunger should stick out (6-6.5mm) and it should be tight. 
Worth checking. 
Michael
 
 
Pulled the end out on the valve stem for a quick check to see if it unscrewed. That looked fine so problem has got to be deeper. Maybe other end. One thing I did notice was the main pressure gauge was reading 25 bar when it was empty. Reg gauge was reading zero. I'll watch Earnest's videos today and then have a go at breaking it down. Another puzzle.