Having just a manual slide on my Aspire, is the air operated slide double acting? Air to extend then another line to retract?
If so, then a simple pilot operated check valve should do the trick, unless the air leak is in the cylinder itself.
Put the check valve in the retract line, then plumb the pilot portion of the valve with an air line Tee'd off the extend air line.
This will allow the check to hold the air in the cylinder preventing bleed off, but still opens for free exhausting when the extend air line is pressurized.
Should be able to get a check valve that just replaces the hose fitting right on the cylinder.
Steve & Tracy with rescues Lucy the Beagle & Willow the Boxador
2017 Entegra Aspire DEQ