I installed a third brake light and quickly realized how hard I have to hit the brakes before the lights come one.

I inspected my setup and the PO installed a separate pressure switch in a T on the brake line out to the rear brakes. The switch located on the master cylinder is not being used.

What I want is for the system to light when I press the brakes (move the pedal) at all, not wait for pressure to build. Is the switch built into the master cylinder a limit switch or a pressure switch?

I may just install a normally closed mechanical switch that is open when the pedal is not being pressed, but I want to ensure the master cylinder switch doesn't do what I want before I go through the effort.

Thanks guys,

Andy

If you're speaking of a 914 or pre-1977 911, the switch is on the pedal, NOT the master cylinder, and it makes contact pretty much as soon as you apply the pedal.

The Cap'n

Hmm, there is a two contact removable switch built into the side of my master cylinder. It looks like this master cylinder came from the factory that way, guess I'll try to find the part number on it.

So the plot thickens......

I'll try to find that original switch on the pedal and use it.

Thanks Cap'n

That's what the factory switch is. . the switch on the master cylinder is the warning switch for brake and master cylinder malfunction. .

The third brake light is usually wired directly to one of the rear taillights with a spade-jumper.

The brake lights are controlled by a separate switch located in the interior right in front of the brake pedal. You have to remove the carpet and pedal board to see the switch.

There is an adjustment on the switch to determine how far the pedal moves before the brake lights come on. You may have to adjust this.

I had the same issue when I installed a 19mm master cylinder. With the new MC, the pedal did not deflect as much as the 17mm plus I finally adjusted the play between the pedal and MC. I had to jam on the brakes to get the brake lights to come on. After adjusting the switch, the brake lights come on now as soon as I depress the pedal.

So what does that switch warn of, I guess what I'm really asking is how mechanically does it work? I would guess it comes on if you press the pedal too far without generating enough pressure? Either way it's on a 67 912 so I don't think it has that warning light even if the master cylinder has that functionality.

Here's my master cylinder, I guess the switch on it only has 1 pole, shows what my memory is worth. You can see the pressure switch T into the rear line in the background.


I found the limit switch in the pedal cluster, thanks as always for your help.



Click to view attachment

That's how I wired it (with a jumper on the taillight). The whole brake system is actuated off the pressure switch right now (from previous owner), I'll be changing it back to the limit switch as originally designed.

Did you rewire your brake switch? Most vehicles have the brake switch on the MC but not Porsche.

Remove your pedal board and you will see the switch down low and behind the pedals a little.

The switch on the master cylinder looks to be a pressure warning switch. We have them on the 914 master cylinders; I'm not sure if an early 912 MC would have one.

There is an extra free-moving piston tied to the two circuits in the master cylinder. If one of them loses pressure, the piston gets pushed toward it. If it moves enough, it closes the contact on the switch and turns on a blinking light in the dash. (I don't really think it's needed; you can DEFINITELY tell when one circuit loses pressure...) You can see the reset button on the switch as well; you push that button to reset the switch.

--DD

From looking at parts diagrams from 67 it doesn't appear there was a MC warning available so I doubt I have a light for it (single circuit MC). It looked like 68 it might have been optional, 69 definitely had it as an option, maybe standard. That's a nice simple way rig a warning light, makes sense from the cutaway views I pulled up on the 69 MC.

Thanks Dave

A 1969 912 would have the pedal actuated switch. Not that it has a dual circuit master cylinder, just like the 914.

The Cap'n

Somebody f-d with that car. Expect to find half-ass work like that everywhere you turn. The brake light switch is the simplest piece of electronics on the car. It took the DAPO WAY longer to install that BS pressure switch than it would have to just fix his brake light switch.