I'll add to the above that if you do not notice a leak and the fluid level is low, don't just top it off. It's low for a reason, and if there is not leak, it's low because you are in need of brake pads. There's really no adjusting the rear since they are disc brakes for the primary system. The drum system back there is only for the parking brake and therefore would have no effect on your brake pedal.
When the vehicle is off if you pump the brake pedal does it firm up quickly?