Hotrod, Give us a call at the number above or email or get us a fax number. We have some info on that system that might help you. One thing you said, the precharge for the 2788-970 accumulator should be 1000 psi. At the 450 when you actuate the brakes you may not have enough reserve for them and you are trying to supply oil through an orifice from the pump which could be causing the hissing. Of course you may have a weak pump also. Bottom the steering and check your pressure should be 1700 - 1800 psi.
Let us know if we can help.

David