Hi, welocme to jA.
Did you bleed the brakes after servicing the rear brakes?
Was the pedal soft at all prior to the rear brake service?
Have you adjusted the rear brakes?
How many miles are on your Civic?
-Rip
Alright, it's possible for a Brake Master Cylinder to become faulty after servicing brakes, if it was on the verge of failure already. But, I do not beleive this to be the case on your situation. If you lost any fluid while servicing the brakes, the system will need to be bled. Air will immediatly enter the system, and air is not 'compressable' like brake fluid is, so the brake pedal becomes immediatly soft. But before we can bleed, you'll need to adjust them. Adjusting the rear brakes is simply, ratcheting the manual adjsuter in the brake assembly, until the drum spins freely but has a slight drag on it. This means that when the brakes are applied, the rear will make immediate contact. If they don't the brake pedal will engage lower to the floor. If after both procedures, the pedal is still soft, than we can venture into master testing. Let me know if you need assistance on either bleeding or adjusting. Thanks,
Lead Technician
Master ASE technician, Honda/Acura aftermarket training, Hybrid training, Adv. level L1