If I had to guess, I would say the piston cups are worn. The 2650 series are fairly simple and straightforward to rebuild, and parts are readily available.

Here's a how-to link.

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=390615


"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.