I started out with a pressure brake switch on my T-Bucket. This was supposedly a pressure switch that activated with 'lower' pressure rather than the standard pressure. I seem to recall that it activated at about 60 psi. The bucket, being light and having good size brakes, did not show it's brake lights until you were somewhat well into the stop with moderate pressure. I remember sitting at a stop light one day and I hear a guy in the car behind shout up "Where's your brake lights?" I put enough pressure on the pedal to show the lights and he then says "Oh, there they are!". That was enough for me - it was time to correct that.
If you are not feeling any spongy pedal, the small amount of air that may be trapped in the switch will be no factor. Remember that in a closed system, the pressure is the same everywhere in the system.
I modified my car to include a mechanical brake light switch, and wired it in parallel to the pressure switch. The mechanical switch is adjusted so that the lights come on just before you feel resistance in the pedal. The whole idea with the second switch being that if one switch failed, the other one would back it up (although the pressure switch would require more pressure to activate). I also wired up my turn signal indicators to reflect exactly what the rear turn signals / brake lights are doing - kind of like a light monitor. This way I can see exactly when the brake lights are activated right on the dash.
Last edited by cucumber1949; 09-14-2011 at 05:54 PM.