I have replaced all the brake lines and hoses on my 2003 Silverado 1500 4Wd. I had no problem bleeding the front brakes, but I can't get fluid through the back brakes. Is the abs system somehow blocking the rear brake circuit?

 

it may have a proportioning valve at the master cylinder...you would need a proportioning valve tool to be inserted to bleed the rear brakes. post a pic of the mc

 

I have one of those tools. When I screw it down on the valve, will it center the piston or do I have to do something along with it?

 

remove the rubber cap...slide the tool into place bleed brakes.
if this doesn't make sense post a pic of your tool and the mc