Hi,
before you take the master cylinder out check if you have fluid to the rear. It could be a blocked inertia valve, if you have the twin master it could be a bleeding technique issue, blocked/crimped lines running to the back wheels, seized cylinders. Rarley do you have to take the MC off to fix a problem.
You can also put bleed nipple in place of the front line outlet and block any movement and that will enable you to isolate the rear system until you get it working properly.
Terry