Dashboard Brake Light Comes On. Why?

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cudaracer

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Lost my manual decades ago, so can't just look it up.....but does anybody know what the round red brake light on your dash means? I am guessing engine vacuum, as it only seems to come on when you are about to stall or stalled. It came on yesterday after driving for first time this year, and the brake pressure felt soft. It went off again and home I went, but I have no idea how to diagnose.

Also, while I am at it, I have been wanting to purchase a devise to check my vacuum. Any suggestions?
Thanks
 
Light usually is low pressure what car?. You just need a quality vacuum gauge to check vacuum. Older the better. I have some if needed.
 
cudaracer said:
Lost my manual decades ago, so can't just look it up.....but does anybody know what the round red brake light on your dash means? I am guessing engine vacuum, as it only seems to come on when you are about to stall or stalled. It came on yesterday after driving for first time this year, and the brake pressure felt soft. It went off again and home I went, but I have no idea how to diagnose.

Also, while I am at it, I have been wanting to purchase a devise to check my vacuum. Any suggestions?
Thanks
Click to expand...

You said the brake pressure felt soft. If it’s the brake light then you have an issue in your brake system
 
65 Dartman said:
You said the brake pressure felt soft. If it’s the brake light then you have an issue in your brake system
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The light comes on periodically. Brakes are mechanical, relying on vacuum. Hard to imagine a mechanical issue that comes and goes. But what do I know.


Car is a 72 cuda with a 318 and 904A
 
65 Dartman said:
You said the brake pressure felt soft. If it’s the brake light then you have an issue in your brake system
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Agreed.

To the original poster.
That light usually only comes on when the brake system pressure is out of balance and the out of balance pressure causes the valve in the distribution block to shift to one side causing the light to come on.

For example, if something is wrong with one end (front or back brakes) it throws the pressure balance off and causes the little valve to shift one way or the other off center.
This makes the warning light come on.
You did say the brake pressure felt soft.
 
You can download the Service Manuals for free. Search for the link.

I'd start with the electrical schematic and determine what switches activate the light. My car is a '73 Dart, so using that schematic as an example, it's the parking brake switch and the switch on the distribution block.

Schematic.PNG


Just saw TrailBeast posted, but yeah, I'd look at that switch and wiring.
 
Yikes, here I thought it was probably related to a large cam and weak vacuum pressure. Got my work cut out for me.
 
cudaracer said:
Yikes, here I thought it was probably related to a large cam and weak vacuum pressure. Got my work cut out for me.
Click to expand...
Shouldn't be that bad to narrow down.
Light comes on if there is a path to ground.
The handbrake switch you can see pretty easily under the dash. Make sure that's not lose or the wire's insulation chafing away.
The warning switch is in the distribution block on the frame rail. A single wire comes up along the inner fender and is wrapped into the harness.
The safety switch works the same way on all variations of the distribution block.
It's explained here: 1970 Brake Hydraulics page 9, Chrysler Master Technicians Service Conference
 
Check the switch on the brake pedal, mine shifted a little to one side and would periodically come on.
 
Does your car have power brakes? Is there fluid in the master cylinder reservoir? When was the fluid last changed/flushed?
 
Go to MyMopar. Download the 72 Plymouth service manual. That manual got there from a guy right here

There are only 3 things that can turn it on

The switching is done in the ground leg, IE the lamp gets power on one side of the lamp, and the switches are on the other leg to ground

1...There can be a wiring short to ground in the switch wiring
2....As mentioned earlier, your parking brake switch can be on/ grounded
3....The differential brake pressure switch can be activated
 
Right, you have a fluid problem, likely. Disconnect the wire at the parking brake release with the light on. If it stays on, it is the combination valve plunger moving to one side or the other, causing the light to come on. A leak, air in the system, leaking master cylinder.

The light has nothing to do with engine vacuum.
 
Demonic said:
Disconnect the wire at the parking brake release with the light on. If it stays on, i
Click to expand...

^^That's right^^ additionally

If it stays on, follow the brake tubing from the master cylinder down to the switch and pull the wire off the switch. If the light goes out, you have a hydraulic problem. If it stays on----and that would be rare---------you have a wiring fault to ground in that circuit.
 
So far, checked fluid, which is good. Took it for another test drive. It started to flicker briefly at idle, then I accelerated and it went off. Did not happen again during that drive. Difficult to diagnose with light on, because it is so brief.
 
cudaracer said:
So far, checked fluid, which is good. Took it for another test drive. It started to flicker briefly at idle, then I accelerated and it went off. Did not happen again during that drive. Difficult to diagnose with light on, because it is so brief.
Click to expand...

Sometimes a good hard stomp on the brake pedal can recenter that switch in the distribution block. (if that's what it is)
I disconnect those lights anyway, because I can tell when the brakes don't feel right and don't need a light to tell me.
 
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