weak brake padal and loss of rpm

The brake booster provides power assist. If the check valve fails/leaks, it creates a vacuum leak resulting in a rough idle and or a harder brake pedal (more effort required to stop the vehicle). I am not sure I under stand "loss of RPM" but by weak brake pedal do you mean the pedal is spongy which could be due to a bad brake master cylinder, contaminated brake fluid, air in the hydraulic system or a lot of brake pedal travel due to out adjustment brakes.