The 3325 and 3328 are normally open relay valves, used to sequence air flow based on pressure levels. This allows the designer to interlock control functions, or allow for time delays in reversing without damage to sensitive pneumatic or propulsion equipment. The 3325 has a single pilot-to-shift input, whereas the 3328 has two pilot inputs to permit pressure return of the spool.
The 3325 / 3328 valves are available in a variety of light to heavy duty spring configurations. Constructed of bronze and stainless steel, these units have 1/4" NPT ports and are rated to maximum supply pressure of 200 PSI (13.8 BAR). Photo shown is of a 3325.