Shouldn't be related to the ABS system, the system is in the "open" mode.
The Prop/Combo valve prolly clicked on ya', is the Red Brake Warrning Light on ? If it is, than the combo valve shut the rear lines off.
Try bleeding the front lines, it might reset the valve. Or you might be able to crack the line (with pressure applied) on the front-out-bound side of the valve.
or
There is a plunger on the side of the valve, I think you hold the plunger "out", or maybe it's, prevent the valve from moveing at all... I don't remember, it's been awhile since I've delt with those.
Anyway, you need to reset the valve, one way or another.