If you have the dual braking system with a line to the front brakes and a separate line to the rear brakes from the master cylinder, then there is valve assemby that the two brake lines from the m/c run into. There is a spool valve in this assembly that sees front brake pressure on one end and rear brake pressure on the other end. As long as the two pressures are the same (as they should be), the spool valve does not move. But if the front or back pressure drops (like if you had a bad leak), the spool valve is pushed to one end of the assembly and shuts of fluid to the low pressure, leaking side. This is a safety feature to ensure you still have braking with the non-leaking circuit. The other thing this spool valve does when it moves is make an electrical contact which lights the brake light to warn you you have a problem.
When you were bleeding the rear brakes, the pressure difference between the front and rear circuit got high enough to push the spool valve over and shut off the fluid to the rear. This explains why the brake fluid stopped coming out when you were bleeding and why the light is coming on. There is a tool to hold the spool piece centred but I have never had to use one. Maybe the pressure applied by the pressure bleeder is high enough to activate the spool valve. First thing to do is get that air out of the rear lines. Try bleeding by just pumping the pedal.
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