I've had the same problem and i noticed that on the s&w housings that the fitting is 1/8" pipe but the actual hole is much smaller than the inside diameter of the fitting. I dont know if this is the problem but on a stock 9" the hole is full size. almost 1/4". Ive raised the vent with a hose and installed a 1/8" screen fitting (f/nos kit)to fix, if that dont work i'm gunna drill out the hole to full size and try that. I did notice that s/w puts the vent in the center area and stock fords have it in the axle tubes.