My brakes have been going soft on me lately, every now and then the brake pedal will sink to the floor and I lose stopping power. Today I had the brake fluid drained and replaced and no fluid loss or air pockets were found. I am guessing it is the master cylinder? but I want to be sure that it couldn't be anything else before I go and buy a new one tomorrow.

 

Does it mainly sink to the floor when you apply the pedal slowly? If so, that's been how bad master cylinder has usually acted for me.

Absent a fluid leak, fluid passing around the piston seal in the master cyinder would be IMHO the most likely thing to allow the pedal to sink to the floor.

 

If there are no leaks from the calipers is has to be the master cylinder. This is assuming that the brakes have been bled correctly