That could be a problem. Most of the ones i have seen had the front of the master cylinder going to the front of the proportioning valve. Where do the lines go from the proportioing valve? the back of the proportioning valve should go to the rear brakes, it should also be a bigger line.
are you running disks in the front and drums in the rear? Or Drums front and back? Do you know what proportioning valve you have? ie. drum-drum or disk-drum. that could also be a problem if you have the wrong valve.
Are you adjsuting the rear brakes out until they just barely drag on the drums? can you confirm that the wheel cylinder is moving by having a buddy pump the brakes?
i installed one of these valves, mine was disk-drum though, for the same price.
http://cgi.ebay.com/ebaymotors/Chevy...Q5fAccessories