I have a 1377 converted to 22 cal (10" barrel)with some mods done to up the power . It seems now the power has dropped a little and so I thought I would chrony it and then pump it and let it sit for 2 full days and chrony it again to see if the valve seals are holding. So,using its favorite pellet (rws Superdome 14.5g) I pumped it 10 times for 3 shots and averaged 500fps. Then I pumped it 10 times and let it sit fore 2 full days and then chronied the shot for 489fps for an 11fps drop.Is this normal? It was for both days about 60degrees out.Also pumping it to 12 gave a readout of 523fps and at 14 pumps -544fps.I usually only hunt with this gun and 13 pumps (537fps) is what I use for shots up to 25 yds max.This gun has the adjustable steel piston with stock factory conical shaped piston cupseal,valve has been ported and polished with possible spring change(dont remember) ,longer bolt probe and trigger tune with spring guide as well as the crosman steel breech and the 10" 22 cal barrel.Looking back on my old figures the gun lost 22 fps at 10 pumps from a few years ago.

... you could also try pumping it, let it rest for a while (15-30min should be enough) to dissipate any heat produced during pumping, and retest.

You may also be seeing results of break-in or some wear on the seals.

you're scared ? go get a dog ... I'd rather die standing than live on my knees

 I guess 11fps isnt much drop over two days regardless then. So maybe the slight drop of 21fps from a few years ago is due to some wear on the seals most probably the piston cup than the valves.