using plain o-rings for a hydraulic piston will require a very close fit
to keep the extrusion gap acceptable for o-rings

my recommendation would be a standard .75 bore , a piston with a wear
band to keep it centered in the bore regardless of load and a polypak
type u-cup seal on the oil side of the piston
In this particular application, a u-cup (although providing perfect
sealing qualities) won't work. I need a flat surface to seal off two
holes in the bottom of a flat bottomed hole.
Thanks.
Ken.