I have a 1989 L98 coupe. I noticed the other day that I have brake fluid leaking. It turned out to be coming from the pressure switch. It's that small white plastic switch thats on top and just a little to the side on the master cylinder. It has a wire going to it. So I whent down to my friendly Chevy dealer and bought one. Well I noticed today that the new one is leaking now. I took the old one a part to see if I could figure it out. Theres a plunger and spring inside and it looks like if the line pressure is to high on the front brakes it lifts the plunger and the fluid will leak out of the switch. The plunger is heald down by the spring and makes contact (ground) with the master cylinder. when the plunger is lifted off it's seat it makes it go to open and that turns on a brake warning light on the dash. So does anyone know if there is anything I can do to fix this or do I need to replace the master cylinder? I rebuilt the front brakes the other day and replace the rubber grommet thats under the front reservoir. I was thinking that maybe if I bleed the rear brakes that could help. I think the rear brakes are working I had the wife step on the brakes and I can't turn the rear wheels , at least by hand I can't turn them.

 

The proportioning valve inside the MC has moved allowing brake fluid to enter the switch port. These switchs are not meant to hold back brake fluid, they are meant to tell the driver that either the rear or front brakes have a major problem. New MC may be in order.

Here's a pic of a bad MC where the proportioning valve is partially "open". When the front or back brakes loose pressure, this valve will slide right or left. When the valve slides enough, the passage in the pic open, allowing the spring loaded switch to open which will set off a dummy "Brake Warning" light on the dash. The spring loaded contact is not "grounded" to the MC valce, it is more like the spring loaded "door ajar" switch. This is a safety feature as it is better that you loose only your front or rear brakes rather than all 4 !!

You should carefully inspect the front brakes since you rebuilt them. If they are leaking, the front hydraulic pressure drops, the proportioning valve "slides" as it is intended.

If your brake warning light did not come on, it is more than likely that the valve is partially open like the one in the pic below, which will not allow the "switch" to extend(yet), but will allow fluid to enter and leak. A pre-warning if you will.




http://forums.corvetteforum.com/c4-t...ng-sensor.html