Hi Stu
My limited experience with my Hydrastar as fitted to my hydraulic brakes on my Tinka seems to indicate that how much braking is applied to the the rotors / tyres seems to depend on your in car brake controller being cranked up to high.
Cheers
Chimo.
PS Re actual psi needed : no idea I just look for the result, a boat trailer that will pull the Patrol up when I operate the in car controller after I set it up.