There is air in your brake system that is expanding under heat and causing the brakes to set and the master reservoir to backfill and overflow. As you just changed the master cylinder, begin by bleeding the MC (you can look up online how to do this) then bleed the brakes in the proper sequence (preferably using a pressure or vacuum power bleeder tool rather than the brake pedal). Power bleeding helps protect the MC seals and will do a better job of pushing/pulling any air locks out of bends and crevices in the system.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now