no fluid going to back brakes

Brake problem...I'm not completely sure how the ABS system works, But
The problem I have is that I am getting no brake fluid to the rear brakes.
Master Cyl. checks OK and I have fluid to the ABS module (2 lines going in 4 coming out).
What would cause me to have no fluid to the rear brakes? Front brakes are fine.

Well not knowing your year/model and engine size...
Id say either your proportioning valve or ABS mod isnt working properly.
Though you could have a collapsed or clogged lines.

Only way to realy check those is with a GM Tech II scanner or other equivalent ABS capable scan tool.

And also sometimes air gets trapped in the ABS module and it needs to be actuated via the ABS scan tool to purge the air.

Hope that helps and Welcome to AF !!