Does it happen more often during hot days? I was experiencing it during the summer, so I bought a replacement MC. But now that the weather is cooler, I don’t experience the problem at all. Is it heat related?
heat will get into your brake lines, making the rubber expand and make the pedal feel mushy. try some steel braided brakelines, they will not expand like the rubber ones.
Here in San Diego it just started getting pretty cool and i have noticed my brakes are hardly mushy anymore.
A simple full brake bleed does wonders ! 
Originally posted by joeygill
A simple full brake bleed does wonders !
Cooked pads, boiled fluid, old fluid, leaky master cylinders…all cause a mushy brake pedal.
Try bleeding the system. SS lines typically don’t help much if at all, even on a track driven production car.
Replacing the master cylinder fixed your problem. Hydrolic / Brake fluid has very high and low temp tolerances.