There's a ready made reduced pressure valve that sits in the saginaw power steering pump here:
http://www.sachserodshop.com/pdf/48.pdf
Does look like the standard valve with a thicker shim fitted though!

Stuart.

1950 Chevrolet 3100 (occasionally working) truck:https://c2.staticflickr.com/6/5490/30684982201_6dc479d18d_c.jpg
1928 Chevrolet AB National Phaeton: https://c4.staticflickr.com/6/5573/30137792083_42886d30a4_c.jpg
1932 Austin Special Sports: https://c8.staticflickr.com/6/5793/30116567183_1520e67c4d_z.jpg