I replaced the rear rotors and calipers, and now I have no brake pressure at all.
Hi there. If the brakes were working fine prior to replacing these parts then you may have air trapped in the system or the calipers may not be functioning correctly. I would first make sure that the calipers are installed correctly. The bleeder screws must be facing up. If not then the calipers were installed on the incorrect sides. If they are fine then the system needs to be bled again. A mechanic would usually use a pressure bleeder to force the air from the system. If the problem persists then it is possible that the master cylinder failed. Due to contamination the master cylinder seals can fail during a bleeding procedure due to debris damaging the internal seals.