88 XJ brake light /sensor question

Ok i put 1 tons under my rig, using a 78 mercury marquis MC in with an adjustable proportioning valve and a 2lb residual valve for the rear brakes, heres my problem.... my brake "idiot" light is on im guessing this is because i plugged the inlet and outlet ports for the rear brake lines on the oem proportioning valve and bypassed it completely in order to not deal with modifying the internals, the brake sensor is mounted between the rear and front input orifices on the oem prop valve, anybody else run into this ? or perhaps theres air trapped around the sensor and i just need to bleed it off? any input would be great! thanks
joe