the brake system may use the hydraulic system in an assisted mode.
if the brakes are acting up whatever is causing the other problems may be affecting the brakes as well.

i'd start with the pressure test on the main pressure at the hydraulic valve, if thats ok then...
move to the auxillary pressure test... if thats ok then...
go back to the priority valve... it may be clean but how does it move inside its chamber? if it sticks at all and i mean AT ALL when you move it in and out then that most certainly could be the cause.
My other thought is some kind of problem in the auxillary valve circuit, all those functions use the auxillary side of the hydraulic system so its understandable you would be having all these issues.