I was gonna try 2.5:1 belt reduction for my rc4wd motor/pump, I think that's what's in the 850. You fashion up the mounting plates with bearings on the pump shaft, get two sets of pulleys & belts to send me one.
(ofcourse I'll pay you for your work, as well as parts)
Did you lubricate your seals before putting the cyl together? Lithium white grease is what I use, the grease will help to prevent any nicks to the seals during assembly. Should be a reasonable amount of resistance there, you want tight seal.