Originally Posted by
DmaxMaverick
The pressure differential valve/switch will trip if either the front or rear pressure is beyond the threshold, causing your brake lamp on the IP. Loosening one rear bleeder allows the front pressure to "reset" the switch. This is why it's important to watch the lamp while applying pressure (and stop as soon as the lamp goes out), so it doesn't set the other way (like the rear brakes are failing).
Other than that, if your brake fluid and system is crappy on the front, there's no reason to think it isn't as bad on the rear. Unless it has been serviced recently, it is equally important. Just because they work now, doesn't mean they always will. Preventive maintenance is way better than repairs.