Gasket in Master cylinder cap keeps popping out

Dear Sir:

Recently I noticed the gasket inside the master cylinder cap keeps to pop out even after I push it back in every time. Brake works normal to me and I did not notice any change from before. BTW, I bleed brake fluid and changed read brake pads about a year ago.

I did not find any leaks from bleeder screw. What could be the cause why the gasket does not stay inside the cap? There must be some leaks, although I did not touch any part of the brake system except brake pad change and brake fluid bleeding. I also unplug brake booster hose a few times for other problem.

Best regards,
Jason