Troubleshooting a brake dummy light stays on

Got a problem with the brake dummy light on my ‘70 standard. It was originally a drumx4 car, with the power front disc upgrade since ‘20. Today I parked on a forward incline and set my parking brake. It was kinda steep and started to roll back a little, so I really floored the pedal.

When I went to leave, the light (not the parking brake light) was on and wouldn’t shut off. Drove normal, with no noises or sense of dragging, but I pulled over to check the master. Fluid seemed a bit low (about 1/2” or so) in the front reservoir, and it wasn’t quite as clear as in the rear. I filled it, but the light remains on.

I had the master off a few month ago, but didnt know about the centering tool (@Royce recommended in another brake light thread). It’s been driving and braking normally for over a month and 500 miles. I’m pretty sure it has to do with the incline braking, but hoping for some troubleshooting help. TIA!

Well it probably means you need to rebuild the brake combination valve. You can probably rebuild your proportioning valve at the same time. Buy the tool from WCCC so the valve will be centered when you are done.

I would pull the master cylinder first to see if it is leaking into the brake booster.

I’ll order the part and check out the master for a leak. I’d always thought the parking brake was all mechanical and not dependent on hydraulics. Can you explain how the incline braking might have caused it…or is it just coincidence?

I think it is just a coincidence. The light being not centered just happened but to get at it you will likely cause the valve to leak so the rebuild kit is needed. Likely the fluid is gone because the valve is leaking but the master deserves a look and will have to be removed anyways.

The little plastic switch on the proportioning valve is spring loaded. Take the lines loose going into it and turn the ignition to on but DO NOT START THE CAR. If the light does not come on, then if all things are equal, the brake linkage from the pedal to the back of the master cylinder may be too long. It’s simple to back off the rod to the a couple of turns and see if the issue goes away. That costs nothing. If your pedal as it sits now is stiff, the master cylinder (even if new!) may be at fault. There are those out there selling manual master cylinders for power brake applications. Question everything. Good luck!

Hell of a coincidence!! I’ve put over 500 miles on it since I failed to center it during assembly - with no braking irregularities or pedal changes…and the light only came on immediately after using the parking brake on that grade. Kinda crazy.

Are these the right ones to order?
• Distribution Block
• Distribution Tool

I would rebuild the Proportioning valve too. It is likely leaking if it has never been rebuilt.

I finally got a minute to get under and inspect. It seems I have a leak between the booster and master (the master was removed when we were working on my guideplate issue in January). The distribution block was rebuilt in 2021, when I had to swap out the 69 booster that was installed in error. Here’s how it all currently looks:

Next steps any different? Still need to pull the master, right? What will I be inspecting for?

Yup, you’ve got a leak out the back of the master cylinder. That looks about like mine looked. When you pull the master, you’ll probably see evidence of brake fluid being sucked into the booster under vacuum. If it hasn’t ruined the booster yet, it probably won’t last much longer. I tried sucking the fluid out of my booster, and continuing to drive it. Lasted about 6 months.

Looks like the master cylinder is leaking into the booster. I would remove the master cylinder and flush the insides of the booster with alcohol. Needless to say you need a master cylinder too.

Alright. So the master cylinder needs to be addressed. Is there a rebuild kit or best to purchase new?

WCCC has two:
Economy for $70 and
Premium for $109.
The description says only cosmetic differences.

Will it be obvious if DOT3 made it into my booster?

I think there is no question you have brake fluid in the booster. The real question is whether the booster is still good.

I would get the brake master cylinder from Dead Nuts On.


Alright. How do I determine the answer to the real question? You mentioned flushing with alcohol. How do I do that? Just hand pump it in and draw it back out?

Is this the right DeadNutsOn MC for rear drums and front discs?

Correct. The rubbing alcohol I get comes in a squirt bottle. You can just squirt it in and use a rag to soak it up. Then hope for the best!

So I’ve pulled the MC and removed the inside mechanism. It looks brand new. Is there a rebuild kit (couldn’t find one at wccc) or just toss it and wait til DeadNutsOn opens and get a new one?

Would I see any evidence of brake fluid infiltration to the booster, and where do I squirt the alcohol

I have a new master for (I assume my 69). It was in the trunk when I bought the car in 18. Markings are 8236KF. No manufacturer on box or instructions.

Cost of shipping & a donation to the forum & its yours.

Looks to be a drum brake master cylinder with the two equal sized reservoirs. It won’t work for disc / drum cars.

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I would just squirt alcohol on the area where the brake master cylinder used to be and on the push rod and on the outside of the booster where the paint has been lifted by the brake fluid. Then wipe it down with a clean rag. Apply some paint to the areas of bare metal on the booster.

Thanks for the generous offer, Mike, but as Royce mentioned, won’t work for my disc/drum.

Royce, do I need to remove the 1-way vacuum valve and introduce any alcohol into the inside?

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Probably unnecessary because if the booster is still good the brake fluid should have never reached that side.