Brake INOP. Replaced gaskets, now what?

[Boilermonkey]

We've replaced the gaskets on the master cylinder and caliper.  Bled the brakes several times and the left side still goes soft after 2-3 flights.  We have not used the parking brake at all.  No signs of fluid.

Any ideas where air might be getting in?

[Fritz1]

try pressure bleeder on reservoir, may take 1-2 qts to purge everything, may need to take brake calipers off and turn upside down while bleeding to get all air out from behind the pistons, turning calipers brings bleed valve up, when pumping brake pedals pump pilot left and copilot lets at the same time to bleed the left brake