Here's an exploded view of the relief valve in a GM pump. The shims go between the end cap and piston housing to decrease pressure on the spring. To increase pressure you remove shims and/or add small washers under the spring.
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Adam
1964 Model 100 w/ K301 12hp and custom hydraulics
1972 Model 149 turned 129 w/ K301 12hp, triple hydraulics, 66 series clone
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