When you changed the master cylinder did you set the height from floor to bottom of pedal at 158mm THEN set master cylinder play
If the brakes stop you then it’s probably more adjustment problem , I think you prob have all the air out
Sometimes there can be no air , but one pump needed to get pistons out and another to brake hard that was a incorrect spring problem on my 11” front brakes
when you leave yours a minute the pistons might retract needing that second pump,
I do prefer TRW Lucas to britpart master and wheel cylinders