What are you trying to do? I'm guessing that you mean the standard hydraulic system that runs things like the loader and lift arms. Here's some kinda general information from a guy who has designed a few hydraulic systems that will help you to evaluate what advice you get...
In every hydraulic system the maximum hydraulic pressure from the pump is immediately limited by a bypass valve so that it doesn't exceed the standard ratings of control valves, hoses, and fittings. I think that's the number you are looking for.
For machines with industrial type and larger HP engines - roughly those 60 hp and up in tractors- those larger engines are mostly used on machines where weight doesn't matter or is an advantage - machines capable of carrying the weight of larger diameter hydraulic cylinders without penalty. The advantage for them is they are able to generate the same cylinder force at lower hydraulic pressures. Lower pressure means that their valves, hoses, pumps, and seals all last longer because of the lower stresses. Larger tractors as well as big backhoes and crawlers typically run somewhere in the 2000 to 2500 psi range. In all probability your tractor can run in that range as well.
Smaller HP machines are often smaller and lighter constructed with a higher center of gravity and so they have to keep weight down on the cylinders, buckets, and loader arms. They make up for smaller cylinders by running higher hydraulic pressure - typically in the 2500 to 2900 psi range. They aren't as concerned with longivity as they are with developing enough force to do the job.
Luck, rScotty
