1990 GMC 1500 sierra no brakes at rear wheels

thanks for answer. "the metering valve/ combination valve" is the proportional valve right? and it is located on side of master cylinder on this truck. what is under master cylinder is the ABS unit RWAL valve with a bleeder steam on it, but no fluid from there when try to bleed eider. and what is the tool to bleed the system? thanks again. I found that at the end of proportional valve have a rubber plug covering a metal cap with a bearing ball on center of it.

Yes, you have the correct valve. The special tool is a L shaped piece that will push in on the rubber plug centering the valve when pushed in. There is a bolt on the valve that is loosend and this tool slid under the bolt and then tightened to hold the tool in that position. Thr tool is about 1" long on the short side and about 4" long the other side and a 1/4-3/8 slot cut in the center of the 4" piece away from the 1" side up about 2" I guess. I you can picture that. The long side will slide under the bolt into the center slot and then tighten down when the other side is pushing down on the centering pin. Hope I haven't confused the heck out of you.

I think I'm getting the idea, but what is the procedure. the center pin has to be pushed in wile trying to bleed the system?. or how that suppose to work can you explain a little more?. obviously i don have that tool. Is there any other way to bleed ? or can you tell me were to buy that tool? Thanks again really appreciate your help.