Re-bleeding the Mastercylinder followed by the rest of the brakes is the best you can do.
If the problem continues it has to be the New Mastercylinder (I had a rebuilt one that never performed well until 1-1/2 years later the pedal went to the floor). Or there is something going on in the part of the Brakes you did not work on.

I suppose there could also be crap in the Brake Fluid if previously the old Fluid was not flushed out.

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84 300D, 82 Volvo 244Gl Diesel