That would be my question - HOW DO YOU KNOW THE REAR BRAKES EVEN MOVE?
If you drained the system at the rear brakes, but see no evidence that the are
being used, I would make sure the cylinders are working.
If the cylinders are really old, maybe the rubber seal is shot (O-Rings?).
Just a thought - Did you buy the car used?
Could someone have installed the incorrect brake shoes? Too small, maybe?
Could the wrong Brake Drum have been installed or switched at some time?
Maybe someone playing around with larger wheel sizes?
Jack the rear end up, take off a wheel and have someone slowly press the brake
pedal and if the piston is working, yell for them to stop - don't want the piston to exit
the cylinder.....Both sides.
Would a bubble in the rear lines cause this effect. I suspect the brakes would
be mushie if you had a big bubble.
BTW - Where is this Proportioning valve located? Do all Geo Metros have one?
DoctorBill