Nissan Repair: 99 altima brake problem, proportional valve, proportioning valve

Question
i have replaced rotors, calipers, hoses and pads...i have worn 3 sets of pads in 6 months...first 2 just basic pads...about 6 weeks ago i got a set for about $50...only the outer pad of the  driver side is worn [in less than 6 weeks] someone mentioned "proportional valve" but i am unalbe to find any thing about that on the web...is there another name for it? could it be the problem...its baffling me because i have replace everything that i know of...any help is appreciated...any idea how much this part is? how hard to replace? where is it located? could this be the problem? is this a dealer part?   the passenger pads were fine...and only one side of the driver side pads was worn completely out...thanks again

Answer
Travis,

I would think the master cylinder is the cause since the check valves are in it.  The proportioning valve us used to divert more fluid to the front than the back.  Also, does yours have ABS.  If so, that is going to need to be checked using an ABS scanner.  If the internals are hanging up on the ABS unit it will cause the pads not to retract.  Sorry for the mispelling as I should have said pads.