Did you check the brake fluid level? When depressing the piston it is common to displace the fluid out and onto the ground causing low fluid level
?? Can you elaborate? I was under the impression that even if you compress the pistons all the way in to the point where they bottom out flush, that with new pads put on, as long as the brakes are not activated without a rotor being between them, that any fluid that is forced out the top of the master cylinder reservoir, would still not leave the MC reservoir in an empty state, low enough to introduce air in through the master cylinder once the brakes are activated again by pushing the pedal?
However I have heard of people who after their pads have worn down some, they will top off their master cylinder reservoir so that it has too much in it when the caliper pistons are retracted to put new pads in, but even then, the amount of fluid remaining should be enough to not empty the reservoir (as mentioned above).
Normally the fluid just feeds back into the master cylinder reservoir, but sometimes people like to open the bleeder screw, thinking that purging the fluid inside the caliper, out of the system to discard is better because it may be more degraded than the rest of the fluid, and in doing so, they might let air into the line. Personally I never do that. If I thought the fluid was bad then I'd flush the whole system instead.
I am unaware of any way to test the Master Cylinder, besides to bleed the system normally, and then if it is still spongy after another bleeding attempt that seems to go well, then the lines can be taken off the MC, those ports plugged (make sure the plugs have no leaks) and then use slight pressure to push on the brake pedal to see if it holds or sinks. Obviously once it is put back together, new MC or not, it will need bled again.
One other thing I would do is examine both front brake hoses in case there is damage.
