Don't know why you want to order a master cylinder you just need to adjust the brake pedal to allow some play in the linkage. of course you need to replace all the other stuff you've ****ed ie pads, sensor and probably fluid.
Because it's most likely the master cylinder sticking. It's a known problem.
You can of course try adjusting the free play for yourself and it may help for a while but you'll end up changing the MC eventually.