Sudden loss of pressure at jets

Pool equipment has been functioning perfectly.  Vacuumed pool on waste for ~15 minutes yesterday. Normally go to rinse for 15 seconds and then filter mode.  Had a senoir moment and went to backwash for about a minute and then to rinse.  I cleaned strainer basket and went to rinse. When I shifted back to filter pressure is down ~40% or more.  I assume I took on air in the system.  My pump and motor are a year old.  But Sta-Rite filter is very old.  So old, I cannot find a air release valve on the filter. To complicate I have a Jandy valve that feeds 4 jet fittings (2 deep end, 2 shallow). When I turn the Jandy, for each direction, the pressure diverts as it supposed to and seems to work properly. But overall pressure delivered to the 4 jets when equally split to all 4 is very low. Any thoughts? Any ideas to bleed possible air from system with no releif valve?