Say that I have a 20T jack. If it has a 4" dia slave cylinder, that
means a (max) pressure of 20 x 2000 / (PI x 2^2) = 3182 PSI. If the
master cylinder is 1/2" dia, then the (max) force on it would be 3182 x
(PI x .25^2) = 625 lbs. If the operating lever has a 12 to 1 advantage,
it would require 625 / 12 = 52 lbs on the lever to generate the 20T lift.

All well & good, but what happens if I put more than 52 lbs on the
lever? I.e., what's the 1st thing to go? A piston seal, I suppose -
but is that right? Anybody ever done it? I.e., tried to squeeze that
little bit extra lift & went too far?

And if it does leak by a seal, is it ruined?

Or is there a pressure relief valve in there somewhere that protects the
seals? Probably not on a HF jack.

Inquiring minds ...,
Bob