Compression Ignition Addict
Join Date: Aug 2008
Location: Madison, WI
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The PRIMARY function of the light is for HYDRAULIC failure. FIRST thing to check is for a failed master cylinder, leaking lines, leaking wheel cylinders, leaking calipers, etc. This is the most critical thing to check, as it's the potentially most dangerous one.
SECOND function is vacuum failure. Even with the vacuum working properly, repeated pumping can overwhelm the vacuum capacity and turn on the light. The pedal should start to get hard when this happens, too. So what you're seeing with this test may not be related to the light coming on at other times.
THIRD function is to indicate you've activated the parking brake. That's a simple switch on the pedal/release assembly. If that switch is acting up, or the wire to it is shorting to ground, it'll turn on the light.
When it goes on, other than the test you did, is the pedal too hard, too soft, or normal? When you have to hold your foot on the brake to hold back the tranny at a stop light, does the pedal slowly drop?