IMO, you may be boiling the brake fluid.Or rather some component in the brake fluid,lol.
Do not use shop brake fluid. Invariably it has absorbed moisture from the air. When you heat the brake fluid, that moisture flashes to steam when you release the pedal at the end of the braking run, and there is your air! The confirmation is that you can run around for days with no issues until that one time you heat up the fluid.
Drain all the fluid and put in FRESH from a sealed container,hi-temp, disc-brake fluid. Then install those "stupid little metal spacers" on the pads between them and the pistons to help divorce the pads from the pistons.
I have been using the 15/16 M/C since 1999, on my KH factory system, and with silicon fluid,with zero problems.And I love it.
Good luck
