Local Hero

On my commute home this evening, I had to make the infamous two-footed panic stop. I practically stood the car on its nose but got it stopped and no damage on either end.

But after that I noticed the warning light illuminated on the dash. It doesn't appear to be fully illuminated. More like 50% -75%. The pedal pressure still feels like it always did. And the car stops just like it always did.

I got home and checked the fluid in the master and all is well. I see no leaks anywhere although I didn't pull the wheels off yet. The master is roughly two years old and a year ago I replaced all the soft parts including both rear wheel cylinders, both front calipers, and all flex hoses. I also power bled the system twice. I've been driving the car daily.

Where should I start looking first? I have a hunch it's the switch on the distribution block on the frame rail. Can it be reset, or can I replace it if replacements are available? Will I need to re-bleed the system if I do replace it?

joe_padavano

The light can be illuminated one of three ways. There is a terminal on the ignition switch that provides ground to the light when the key is in the START position, which is part of the "lamp test" function. There is a pin switch on the e-brake pedal that provides ground if the pedal is not fully released. And finally, there is the differential pressure switch in the combo valve that provides ground if the pressure in the front brakes isn't the same as the pressure in the rear brakes. If the light remains on, pull up on the e-brake pedal to be sure that isn't the problem. If not, then it is likely the differential pressure switch. Disconnect the wire to the combo valve to see if the light goes out. If it does, you have pushed the piston in the combo valve off center, which means there is a problem in one half of the brake system. Something is leaking an needs to be fixed. If there are no obvious leaks, pull back the dust boots on the rear wheel cylinders and look for wetness.

Local Hero

Thanks Joe for the information.

Checking the e-brake was the first thing I did.

Upon a thorough inspection, no leaks were found.

The switch was "stuck."

A few decent hits to the block with a hammer and the light is out.

Maybe I broke it.

joe_padavano

A couple of things to be aware of. First, if you have the original cast iron combo valve, the internal piston gets stuck because of rust in the bore. Consider a brass replacement.

Second, the piston doesn't move unless there's a pressure differential from one side of the piston to the other. If it moved during a panic stop, you've got a brake issue somewhere. When the piston moves all the way to one side in the combo valve, it closes off the port to the half of the brake system that was leaking. Now that you've recentered the piston, keep an eye out for new leaks. Also, you have have an internal leak inside the master cylinder. If seals in one side of the MC are allowing fluid to leak past, there won't be an external leak, but you'll still loose pressure in half of the braking system.

Local Hero

Thank you. I'll stay on top of the situation.