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Old 07-27-2018, 09:57 PM   #1
Tkrysl
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Brake light on the dash- help needed

Okay guys I have my first cry for help.

Out of all the things that I had to do on my 72 C15 to get it to pass safety inspection I'm down to the final thing and I cannot figure out how to get this brake light off my dashboard.

The safety inspector failed me because he said my "master cylinder gasket was swelled" and I have the red light on the dashboard. He told me I needed a new master cylinder to get that light to go off.

I replaced the master cylinder with a new one put new rotors and pads and calipers and shoes on the back. I'm a Noob at this so if you have any advice on why that red brake light is on I am all ears because I have to have this thing reinspected Monday morning.

Thanks!
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Old 07-27-2018, 10:07 PM   #2
AussieinNC
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Re: Brake light on the dash- help needed

There is a brake bias valve that has a trip switch built into it...the switch has tripped due to brake fluid loss on one of the circuits...

Follow your brake pipes down from the master cylinder until you find a small cylinder like valve mechanism with brake pipes going in and out..
There will be a wire on a switch that screws into the body of the valve ...remove the wire and the light will go off...

BUT...remove the switch and you will hear a click...that's the bias valve re-seating...screw the switch back in and replace the wire..

The light should stay off...


Last edited by AussieinNC; 07-27-2018 at 10:18 PM.
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Old 07-27-2018, 11:26 PM   #3
Tkrysl
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Re: Brake light on the dash- help needed

If I unhook that wire, the light does go out, but the guy won't pass me without the wire hooked up.
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Old 07-27-2018, 11:42 PM   #4
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Re: Brake light on the dash- help needed

If donít want to re-set it, remove the bulb in the dash.
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Old 07-27-2018, 11:49 PM   #5
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Re: Brake light on the dash- help needed

Like the other guy said, remove the switch from the per portioning valve , press down n hard on the brake pedal and it should reset its self. If not, itís stuck.
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Old 07-28-2018, 12:01 AM   #6
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Re: Brake light on the dash- help needed

What the switch does is ground the wire that plugs into it and this provides a path to ground for the light. If you disconnect the wire the light should go off unless the wire is shorted to ground in another spot. You could check this easily with a meter.

here is a picture of the valve and the wire that we are talking about.

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I have read that a really hard stomp on the brakes will reset the bias valve too.

I forgot to say. to the forum.
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Old 07-28-2018, 11:22 AM   #7
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Re: Brake light on the dash- help needed

Consult your service manual. When you bleed your brakes, you're supposed to make a little bracket that bolts on to the master cylinder and depresses the switch. Bleeding your brakes without said bracket doesn't always force the switch hard over to one side but this is what happens when it does.

You just need to recenter the switch.
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Old 07-28-2018, 11:52 AM   #8
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Re: Brake light on the dash- help needed

If you know your brakes are working to spec and the only think keeping you from getting your inspection sticker or creds is this guy that will not pass due to the light, pull the wire out of the P-valve and tape the contacts so that you can plug it back in and the light will stay off.

Then go get your inspection pass.

Then deal with the light on your own time without this time pressure.

It's a 50+ year old piece of hardware that the owner likely knows better than the inspector. Screw him. Do what you have to do to make him see what he needs to see.

Hth,

-klb
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Old 07-28-2018, 01:21 PM   #9
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Re: Brake light on the dash- help needed

Stomp the brake pedal really REALLY hard. The light will go out.
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Old 07-28-2018, 01:45 PM   #10
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Re: Brake light on the dash- help needed

While it's running?
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Old 07-28-2018, 05:31 PM   #11
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Re: Brake light on the dash- help needed

Remove the wire from the switch

Unscrew the switch and remove it

Stomp a couple of times on the brake pedal with the engine running...

Replace the switch and connect the wire

The light should have gone out...

If it still lights after the above process, the wire might be grounded by being pinched perhaps when work was done..or...the switch pin that sticks out of the end of the switch that screws in may be bent of broken off...grab another switch and try again.
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Old 07-28-2018, 10:37 PM   #12
Tkrysl
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Re: Brake light on the dash- help needed

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Originally Posted by AussieinNC View Post
Remove the wire from the switch

Unscrew the switch and remove it

Stomp a couple of times on the brake pedal with the engine running...

Replace the switch and connect the wire

The light should have gone out...

If it still lights after the above process, the wire might be grounded by being pinched perhaps when work was done..or...the switch pin that sticks out of the end of the switch that screws in may be bent of broken off...grab another switch and try again.
Thanks! It didn't quite work the first time, but I tried a second time and pushed the pin in and out a bunch of times with my finger, twisted it back in, hooked up the wire, and no light!

You guys are awesome.
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Old 07-29-2018, 01:30 AM   #13
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Re: Brake light on the dash- help needed

Quote:
Originally Posted by AussieinNC View Post
Remove the wire from the switch

Unscrew the switch and remove it

Stomp a couple of times on the brake pedal with the engine running...

Replace the switch and connect the wire

The light should have gone out...

If it still lights after the above process, the wire might be grounded by being pinched perhaps when work was done..or...the switch pin that sticks out of the end of the switch that screws in may be bent of broken off...grab another switch and try again.
Aussie, I've never seen it stated anywhere (manuals, I mean) to remove the switch before stomping on the pedal.. Where does that come from and what is the benefit of doing that? I've had issues with these lights from time to time also, and have gotten them sorted out, but never removed (or thought to remove) the switch itself. Just curious - if I missed a step somewhere and this would have made it simpler and more likely to work when stomping on the pedal to reset via pressure, I'm interested, thanks.
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Old 07-29-2018, 05:31 PM   #14
AussieinNC
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Re: Brake light on the dash- help needed

The small pin that sticks out of the switch drops into a slot machined into the valve itself and locks the valve in the fail over position...

Some switches have the tip of the pin rounded off when manufactured,,,which will allow the pin to be forced back over the lock groove if the pedal is stomped on hard enough...

I have always felt that removing the switch takes only a few minutes and allows the valve to center itself again with only a light application of the pedal.

There is no chance of any fluid loss...

I have had vehicles come to me with the brake dash light stuck on...removing the switch usually show the tip of the plunger snapped off...new switch will fix it every time.

These bias switches are a necessary item and should not be omitted or disconnected...they are a quick alert to a failed brake fluid circuit.

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Old 07-29-2018, 05:52 PM   #15
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Re: Brake light on the dash- help needed

Is that switch setup any different in the 4 wheel drum brake setups that have the distribution block instead of proportioning valve?
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Old 07-29-2018, 08:20 PM   #16
AussieinNC
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Re: Brake light on the dash- help needed

Drum / drum systems still use split circuit designs and I believe most have a bias valve trip system built in...

I just checked my original factory 67 longbed I am working on and it has a proportioning valve like valve immediately under the master cylinder...with a switch built in...

I believe the valve in this scenario trips when a hydraulic circuit fails on either front or rear brakes.

A true proportioning valve is in a Disk / drum or disk / disk system to limit the pressure applied to the rear system to prevent lockups etc.

The alert switch is in these later style systems again to alert of hydraulic system leaks.
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Old 07-29-2018, 09:25 PM   #17
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Re: Brake light on the dash- help needed

Quote:
Originally Posted by AussieinNC View Post
The small pin that sticks out of the switch drops into a slot machined into the valve itself and locks the valve in the fail over position...

Some switches have the tip of the pin rounded off when manufactured,,,which will allow the pin to be forced back over the lock groove if the pedal is stomped on hard enough...

I have always felt that removing the switch takes only a few minutes and allows the valve to center itself again with only a light application of the pedal.

There is no chance of any fluid loss...

I have had vehicles come to me with the brake dash light stuck on...removing the switch usually show the tip of the plunger snapped off...new switch will fix it every time.

These bias switches are a necessary item and should not be omitted or disconnected...they are a quick alert to a failed brake fluid circuit.

Copy, this makes sense - am assuming you also need to re-center the switch pin before reinstalling it onto the valve? I can see where no fluid loss would be possible - are there O-rings on the valve itself on either side, i.e. separating it (and air) from both circuits, i.e. a dry switch connection? Very interesting, thanks for posting this, have never heard of this process before, but it makes sense to me. Wonder why the manual doesn't call out this as normal procedure when resetting - it has the funky specialty tool, etc outlined in every manual I've seen.
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