Hi just reading your question about spring replacement, I’ve worked on a few Airsporters and found it very frustrating, I’ve tried different springs, polishing pistons, and found the biggest problem is the parallel taps.
The mk1 & 2 are fine but mk3 onwards it’s a engineering disaster, I was determined to fix the problem and have come up with the solution, I machined 2 groves in the tap and fitted 2 ‘O’-rings either side of the pellet hole. I can now decock the rifle with the tap in the open position and it holds the pressure, now all the power is delivered to the pellet.