1993 Chevrolet Astro dash board brake light on

Tiny
GMS01
  • MEMBER
  • 1993 CHEVROLET ASTRO
  • 4.6L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 198,000 MILES
Dash board brake light on when vehicle is operating, but goes off when brakes applied. Front end vibration and hum/rumble when vehicle is at speed, but goes away when brakes applied. Can find no obvious brake fluid leaks. Thanks for your help.
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Monday, September 8th, 2014 AT 9:50 AM

3 Replies

Tiny
CARADIODOC
  • EXPERT
There's three switches that will turn on the red brake light. Any of them could cause the light to go off too, but the secret is keeping it on so you can diagnose the problem. The first is the parking brake. Pull that fully-released. If the light goes off, suspect the main cable is stretched or one of the rear cables is rusted in the partially-applied position.

Next is the low fluid warning switch, if you have one. Unplug it or add a little clean brake fluid. If it's real low, don't fill it. Check for worn front brake pads.

The last one is the pressure-differential switch. That's on the combination valve on the frame rail under the master cylinder. Unplug that. If the light goes out we'll have to discuss what to do next.
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Monday, September 8th, 2014 AT 9:26 PM
Tiny
GMS01
  • MEMBER
Seems to be the last of the three options.
What do I check next?
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Wednesday, September 17th, 2014 AT 9:23 AM
Tiny
CARADIODOC
  • EXPERT
That switch is turned on by a valve that shifts off of its centered position when there is an imbalance between the front and rear hydraulic circuits. In other words, it's due to a leak or some other reason pressure didn't build up as quickly on one part when the brake pedal was pressed. That valve can also be tripped when not following proper procedures when pedal-bleeding the brakes after some other service.

On Ford products that valve is not spring-loaded and is REAL frustrating to center it again. A special tool is made to hold the valve centered, ... As long as you know ahead of time you're going to be doing something that can cause it to shift. On Chrysler and GM vehicles that valve is spring-loaded, and it will usually reset itself but sometimes they stick in the corrosion that forms in the housing. A good jab on the brake pedal usually gets them freed up so they can reset. If that doesn't work, and you know, for example, that there was a leak in the rear hydraulic circuit, once it's fixed you can open the front circuit anywhere, then slowly press the brake pedal until the valve snaps free and the light goes out. At that point you have to hold the brake pedal right there until a helper closes whatever was opened to do this. If you let the pedal up before the circuit is closed, you'll draw air in. If you loosened the steel line at the master cylinder, if you're careful and patient, you can bleed any air out yourself by slowly pushing the brake pedal about one third of the way down, holding it there for half a minute to allow any air bubbles to float back up, then allowing the pedal to release quickly. The fluid returning to the master cylinder will wash any bubbles back up into the reservoir.

The issue now is why did that valve move? Was any brake work done recently or is there a leak somewhere?
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Wednesday, September 17th, 2014 AT 9:27 PM

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