The pump has an internal pressure relief valve that is set to open at 1100-1200 psi. There is no adjustment on this, but you should be able to lower the pressure by replacing the relief valve spring. This spring is inside the flow control valve and you can get to it by removing the ball seat.
Since reproductions of the Mustang II rack are commonly used on Chevy powered rods (basically the same pump as Buick) I would start by asking the aftermarket rack vendors what spring to use.
Ray
Bookmarks