ok, on my 88 3/4 tom 4x4 sub i had the bracks done about a year ago when i put it back on the road. new front pads a new rear line, drum, wheel cyls, shoes and springs. in the summer i replaced the pads and calipers do to them starting to sieze. now i have noticed that my front brakes are locking up because my rears seam to do nothing. I have been thinking that this might be my proportioning valve. i havent had the time to look at the system much due to my new job, but the cyls done seem to be leaking. could it be anything elce?