Brake Light Sending Unit
 

1980 911SC. My brake lights are out. Easy enough to test the bulbs with the emergency flashers. I can look at fuses. Is there a way to test the sending unit on the master cylinder? It would be nice to be sure before ordering a part.

sure - do you own a multimeter?

Yes. I do. Is there a way to check across the unit for resistance? Do I need to remove the unit from the master cylinder?

Those brake light sending units are pretty much crap they cant take the current put through them (search on the forum) do yourself a favor and install a mechanical brake light switch and bypass the stock hydralic switch I did 26 years ago and havent looked back.

Just short the wires that attach to the switch. If that makes the lights turn on, your switches are bad.

If it is the switch...
 

Factor in rebleed on brakes unless you are very good (fast) at R&Ring tapered thread items in a fluid system. If it is the original 30-year old MC, you might consider a new unit - the factory switches on the new MC are superior to the VW items, and at the cost of the switch you might just pitch good money after a new MC to remove any doubt about the seals, etc.

Great time to change out your fluid but as I said the hydralic switches fail fairly quick now days.

My repair didn't go the way I planned. Duh.

I replaced the sending unit about 18 months ago, so I was surprised that it failed so soon, but the forum says that they do fail more often than you'd hope for.

This is a daily driver and I'm commuting by using hand signals to keep from getting ticketed or rear ended, so I took it to a local independent. Nice guy - charged me $40 to put the new unit in - total - parts and labor. And he did it on the fly. I was in and out in 20 minutes. Brake lights work now.

Here's the new problem. Now the brake indicator light is coming on the dash. Stays on. I can shake it off by cycling the E-Brake but it comes back as soon as I touch the brakes. So did I get a bad sending unit or does it seem that there is more going on here?

New Master Cyl within 30K miles. No apparent leaking anywhere and reservoir is steady full.