Canopy seal who does not deflate

I was called to fix some  squawks on a Fouga .
Main concern of the owner was smoke in the cockpit.
I ran the engines to 22,500 rpm on the ground for 5 minutes, checking at the same time the pressurization and I could not reproduce the problem, pressurization working perfectly.
Owner, his son and me did the servicing of the aircraft, then Sunday night we did the test flight.
After take off, with pressurization on, we had no smoke but ice particle and condensation due the high humidity coming out of the ACM diffuser, so I increased the temp control to full hot, water came out of the vents, then after 1 minute, back to medium cold and everything was normal. A couple of aerobatic evolutions, all check fine, back to earth.
Out of the cockpit, and closing the canopy : the canopy seals stay inflated.
I checked the canopy selector valve, look like it's working.
After 20 minutes the owner could close the canopy.
That evening looking down the runways lite, I called my friend Didier and talked about the problem.
The outlet vent for the canopy seals could be plugged.

and yes it was. Plug removed, problem solved.