SWB master cylinder brake light switch issue

[mrm930]

1967 911S. Well, after replacing the plunger style brake light switch with a new one - the brakes lights stay on. Switch works - pre-check before installation. I hooked it up to the wire harness - pushed down on the plunger and the brake lights came on - released it, and they turned off. So the switch and the wiring is good. I did notice that the plunger on the new switch is longer than the original - probably will start there.

I will remove it again tomorrow and check the diameter of the plunger - thought that when installed it - pushed in and did not release. So starting there - sand down the diameter just a bit. If that doesn't work - then I will slowly start reducing the length.

My question is: has anyone had the same issue with a new switch?

thanks,
Mark

[edmayo]

Are you running the original single circuit master cylinder?

[mrm930]

yes, its the factory original. single circuit

[edmayo]

Can you take picture of the uninstalled switch?

[mrm930]

pics of switches

[mrm930]

not as familiar with these single swb master cylinders as with the ones in the 70s and 80s. Okay, to activate the brake switch I assume a pin from inside the master cylinder pushes against the plunger of the brake switch. I put a smaller diameter drill bit end against the cylinder pin and had some one push on the brake - nothing. No movement inside the master cylinder. Could this pin be seized or is it something else? Hate to tear this apart if it is not necessary. would have to rebuild it and re-bleed the brakes, and the brakes themselves work great.

suggestions?

[edmayo]

Those are not brake light switches for a single circuit master cylinder. Those are for dual circuit m./cyl. to activate brake warning light.

[mrm930]

yes - just figured it out after posting pics. So then my brake light switch is the bolt on switch adjacent to the brake pedal. However my bracket is not set up for that. So the head scratching continues. My car is a 67, however the specific master cylinder relates to a 68 (looking at PET diagrams). So I my have to fabricate a brake so that the switch would work

[edmayo]

How long you been running without brake lights??? There's no way you've had brake lights! So now you don't have a single circuit master cylinder, you evidently have a 68 master cylinder which is a dual circuit master cylinder. Now that's a good upgrade (did it on my 67) but that requires a later pedal cluster with the mechanical brake light switch. Or as I did install the correct 67 pressure switch in the brake line where line pressure activates it. On the master cylinder picture you'll see the original pressure switch pointing up from the rear most brake line. Also I installed a BMW reservoir with the partition inside, in the original reservoir location . Used a spacer for the longer bolt for the larger diameter reservoir.