since both your bulkheads are(should be) vented to the cockpit, your kayak can be turned into large air pump. to check the rear, really cinch down the front hatch, maybe a web clamp over the whole thing, maybe even some saran.
wet the cockpit rim, tie the waisthole of your sprayskirt w/ rope and tightly spread on the yak. Violently push up and down while spraying soapy water around the rim. Leaks may show.

It'll depend on how big the vent holes are on how well the method works. make big ones(1/4in) and plug later w/ tiny holed dowel or surgical tube.

Another way is to grease the gasket and push down hard, does it very very very temporarily hold or just quick release. if quick releasem maybe put some paper strips around and see which pull out easiest to get the problem area.

Or just do it the best way and silicone in a gasket seat. perfect match and no brainer.

-mick