Brake light comes on when accelerating hard(ish)

[DAMatt]

So here I am after a crappy day at work, taking the Charger out for a drive through the hills & forests around the city to vent. Some time after I started driving hard, I saw the Brake light flashing when upshifting from 1st to 2nd @ 6k rpm. Then, I saw it coming on and staying on when I was more than 50% on the throttle. First suspicion was low brake fluid level, babied it home, and sure enough it was low on the front brake fluid reservoir. If it matters, I have the drum brake master cylinder, 73-76 11" discs, and the factory booster.

Now comes the wtf part, filled the reservoir to the brim, and took it for a drive - same symptoms. No puddle under the car. Brakes were working fine all the time.

Anybody care to explain exactly how the brake warning system works? Could it be an electrical thing, and not related to the hydraulic circuit?

 

[Kern Dog]

First, the master cylinder is wrong for the application.
Second, the warning light is activated by either a pressure loss or a poor connection at the proportioning valve.

 

[R413]

Or the park brake pedal not staying all the way up, the park brake switch bad or out of adjustment, or the signal wire getting shorted to ground.

 

[DAMatt]

I am meaning to do a whole brake upgrade over the winter, and will of course get the correct master cylinder. BTW, I've been long asking myself, why is it important to have the correct one? I mean, it's just a tank that holds liquid to be delivered where it needs to go, right? What can happen if you have discs & drum master?

So you guys rather lean toward an electrical issue, right?

 

[Kern Dog]

If you removed the residual pressure valves, the only issue is the limited volume the drum master cylinders have. If you keep checking the fluid level to make sure it is topped off, you'll probably be okay.
The disc MCs are larger and hold more fluid.

 

[DAMatt]

amm... what does a residual pressure valve do, and where is it? Back when restoring the car I just bought a master cylinder for a 1968 Charger; it fit so it went on the car... no need to scold me on my ignorance, already done that

 

[Kern Dog]

Drum brakes are designed so that the drum shoes are just slightly off the surface of the drum. It takes a little bit of pressure to close that gap.
The RPVs are used in 4 wheel drum brake cars to keep a small amount of fluid pressure in the lines so that the drums can react quicker. Disc brakes react faster so they do not need the residual pressure valves.
If your car was already modified before you bought it, the previous owner may have removed the RPVs. They are pressed into the master cylinder just inside where the brake lines connect. I have no pictures on file but if you do an internet search or "residual pressure valve", you may find proper pictures and diagrams of them.

 

[493 Mike]

Drum brakes are designed so that the drum shoes are just slightly off the surface of the drum. It takes a little bit of pressure to close that gap.
The RPVs are used in 4 wheel drum brake cars to keep a small amount of fluid pressure in the lines so that the drums can react quicker. Disc brakes react faster so they do not need the residual pressure valves.
If your car was already modified before you bought it, the previous owner may have removed the RPVs. They are pressed into the master cylinder just inside where the brake lines connect. I have no pictures on file but if you do an internet search or "residual pressure valve", you may find proper pictures and diagrams of them.

To expand on your answer: wheel cylinders use a "lip" style seal which requires a pressure to apply/seal, the RPVs keep a little pressure at the lip to prevent seepage. Most calipers have a square cut O-ring which applies constant pressure to the caliper bore at all times, preventing seepage and it also causes the piston to retract a slight amount to prevent pad wear/drag. This action is similar to the springs retracting the brake shoes in a drum system.
Mike

 

[pnora]

So here I am after a crappy day at work, taking the Charger out for a drive through the hills & forests around the city to vent. Some time after I started driving hard, I saw the Brake light flashing when upshifting from 1st to 2nd @ 6k rpm. Then, I saw it coming on and staying on when I was more than 50% on the throttle. First suspicion was low brake fluid level, babied it home, and sure enough it was low on the front brake fluid reservoir. If it matters, I have the drum brake master cylinder, 73-76 11" discs, and the factory booster.

Now comes the wtf part, filled the reservoir to the brim, and took it for a drive - same symptoms. No puddle under the car. Brakes were working fine all the time.

Anybody care to explain exactly how the brake warning system works? Could it be an electrical thing, and not related to the hydraulic circuit?

With the light NOT coming on when using the brake tells me the hydraulic part is working ok. With the light coming on with acceleration tells me something is moving. As said in post 3 check the emergency brake, switch, and wires. I would disconnect the switch and drive it. If ok than that was your problem. If not unplug the wire at the distribution block and drive it.

 

[toolmanmike]

I am meaning to do a whole brake upgrade over the winter, and will of course get the correct master cylinder. BTW, I've been long asking myself, why is it important to have the correct one? I mean, it's just a tank that holds liquid to be delivered where it needs to go, right? What can happen if you have discs & drum master?

So you guys rather lean toward an electrical issue, right?

Before you get into a complete brake overhaul, check the emergency brake like R413 mentioned. Never overlook the obvious.

Or the park brake pedal not staying all the way up, the park brake switch bad or out of adjustment, or the signal wire getting shorted to ground.

 

[R413]

Start with the simple things. You are driving the car and not complaining of braking problems. You will do this “whole brake upgrade” and have the same problem with the light.

 

[Sixpactogo]

My guess is like Pnora mentioned is the switch on your proportioning valve probably getting a partial ground. I had that happen on my 68 Charger. Brakes were fine but the light started flashing dimmly on and off. I replaced the switch and the problem went away. If you don't want to deal with it immediately, you could just unhook the switch at the proportioning valve until you do the upgrade.

 

[70 sublime]

Just lift the park brake pedal a little next time the light comes on to see if it will go off

 

[RemCharger]

Just lift the park prake pedal a little next time the light comes on to see if it will go off

Exactly.