I can tell you a little about this from my steering box adventures. The pressure side of a stock Mopar box is between 700 and over 1000 psi for B and C bodies, actually the pump spec. That's part of why they feel like novocaine in your arms. The return side is very low pressure, like 4 psi back to the pump. That's why you can remove the cap while idling. If the seals in the steering box wear - and they will eventually - the high pressure side leaks through internally, putting too much fluid around the input shaft. Eventually, you'll need to replace the seals.
