Chevrolet Repair: 1988 C1500 brakes, flex hoses, rear brakes

Question
I changed pads and shoes, installed repair kits in rear calipers, and replaced front and rear flex hoses.  Also disassembled and cleaned out the master cylinder (would have installed repair kit in it too, but the kit I bought was the wrong size).  When I bled the brakes the front brakes seemed to bleed fine, but I couldn't get much flow from the rear brakes (used a hose and bottle to bleed brakes).  Brakes seem to be okay, but I'm concerned that the rear brakes still have air in them.  Any secret to bleeding the rear brakes?  There's another cylinder of some sort mounted below the master cylinder, and what looks like some kind of control module mounted to the side, and hydraulic lines from the master cylinder interconnect with these before running elsewhere.  The cylinder has a bleeding stem on it, which I couldn't get anything to come out of, and the module has some sort of valve stem on it, though I didn't try to open it.  I figured these have something to do with why I can't seem to get a good bleed on the rear brakes.

Answer
Hi Chris ... that is called a leveling valve...it balances brake pressure from front to reat.now when you bleed the brakes you need to bleed in the proper order. always start at the longest line first...which is the right rear...while bleeding if you look on the balancing valve there should be what looks like a button. that is the centering valve you need to hold that in when you first start to bleed, that will allow fluid to go to the rear. bleed right rear then left rear then right front then left front...that valve should stay centred once you do the rears.... hope this helps