Brake pressure warning switch Options

[SVG223]

In the middle of rebuilding/bleeding the brakes and the parking brake warning light is flashing while doing test drives and won't shut off. After checking the parking brake switch I do a little research and read about the brake pressure warning switch on the master cylinder which has a reset button. I'm thinking about how often I might have to reset the thing before I'm done with the brakes. What about cutting the wire to the switch until I decide to rebuild/replace the master cylinder, then reattach it? I don't think I need a flashing light to tell me that I have low brake pressure, that should be obvious.

[Cairo94507]

Please do not cut the wire. The brake light warning switch is easy to get too once you have done it once it is a no-brainer. (IMG:style_emoticons/default/beerchug.gif)

[SVG223]

Thanks for letting me know that it was easy to get to, Cairo94507. I have made no attempts since discovering that it wasn't the parking brake switch.

[Cairo94507]

OK - the master cylinder is under the car, right behind the front wheels, below the gas tank. You can access it by jacking the car up and if you still have the steering rack protector plate installed, remove that (4 bolts) and the master cylinder is on full display. (IMG:style_emoticons/default/beerchug.gif)

[SVG223]

(IMG:style_emoticons/default/tiphat2.gif)

[bkrantz]

Original switches have two terminals. You should be able to pull the leads if you remove the left wheel, since the switch is on the left side of the MC.

You can remove the switch without breaching the brake fluid system.

[SVG223]

I pulled off the wire and it stopped flashing, which was good enough for me.

[GregAmy]

It is a pin switch that works on the central shuttle valve. That shuttle valve moves from its center based on differential pressure between the front and rear circuit. If one circuit has pressure and the other does not (or has significantly lower pressure) then the shuttle valve moves toward the low side, activating the pin switch. The light comes on, telling you one of your circuits has low brake pressure.

Once you fully bleed both circuits, the shuttle valve auto-centers, the pin switch is deactivated, and the light goes off.

So if you're driving the car and the brake light is coming on, then either you have a damaged master cylinder or one of your brake circuits is faulty.

BTW, technically there should be no brake fluid in that center of the master cylinder body where the shuttle valve is. You can remove/replace that switch and not have to re-bleed the system. But, some fluid may leak by the seals so that's why many switches will have a weep hole. but if you pull that switch and there's a lot of fluid in there then you have a faulty master cylinder. - GA

[Dave_Darling]

Many of the switches are latching, so even when the shuttle moves back into place it still grounds the single contact (or bridges the two contacts together). Those must be reset by pressing the button on the switch--usually it's covered by a rubber cover.

Some switches are not latching, so when the imbalance goes away and the shuttle re-centers itself the light goes off. I believe that those are replacement switches.

--DD

[GregAmy]

Many of the switches are latching, so even when the shuttle moves back into place it still grounds the single contact (or bridges the two contacts together). Those must be reset by pressing the button on the switch--usually it's covered by a rubber cover.

Ah hah! I've often wondered about that, as I've read here about that button but none of my cars (or replacement parts) ever had them. I learned something cool today...

And that latching makes total sense, since you want the driver to know that there's a mechanical problem to be addressed. I guess it was easier (and cheaper) for manufacturers to make the switches simpler (and cheaper) and hope the driver figured it out... - GA

[Dave_Darling]

I've had a loss in pressure once due to boiling the brakes at the track.

I absolutely did not need the light to tell me that my brakes were f***ed. When the pedal went to the floor (at over 100 MPH), the light started blinking and my first thought was "No s**t, Sherlock!"

--DD

[GregAmy]

(IMG:style_emoticons/default/happy11.gif)

(IMG:style_emoticons/default/driving.gif)