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Greg Tubbs  | 

Hello David
The adjustment on the 553 auto fill valve is the brass screw on the top of the valve in the center of the dial. this is the only way to increase or decrease the system pressure.
The grey knob on the bottom of the valve is an isolation shutoff for servicing purposes. The grey knob should be screwed out all the way from the body. Turn the knob towards the minus direction. When the grey knob is in the fully open position, the flow cartridge can move freely against the spring and diaphragm to the correct pressure setting.