Front Brake Light Not Coming On

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fgr33n
Posts: 3
Joined: Sat May 26, 2018 8:49 am

Front Brake Light Not Coming On

Post by fgr33n » Sat May 26, 2018 9:33 am

Hi all, I just found this forum while searching for info on a brake light problem.

I have a 1975 R75/6 that has been sitting a lot for the past couple of seasons. On firing it up this spring, I've found that the lever for the front brakes doesn't activate the brake light. The brake pedal still turns on the light, so I know it isn't something simple like a bulb. Also the actual brakes seem to be working fine.

After searching through all the posts on front brake light problems it sounds like there is a hydraulic switch on the master cylinder under the gas tank that is probably faulty? Is this true?

Also, it sounds like I can test the switch by jumping across it-- if the brake light comes on then the switch is bad. Does that sound correct?

And finally, some have replaced this switch with an inexpensive unit for old VWs from an auto parts store. Does that also sound true?

Or is there anything else that could be causing this problem that I should check?

Wiring/electrical is my weakest area, so I wanted to collect some input (and confidence) before trying to fix it.

Any and all advice is much appreciated.

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melville
Posts: 1401
Joined: Tue Aug 03, 2010 8:32 am

Re: Front Brake Light Not Coming On

Post by melville » Sat May 26, 2018 10:01 am

Great to have another R75/6 rider on here. The answers to your questions are mostly 'yes.' I can't confirm that the switch is also a VW part, but that's not a stretch by any means.

If you have a multimeter, you can also test the switch by checking for continuity across the terminals when the brake is applied. Set it to Ohms, hold the probes one on each tab, and apply the brake. Resistance should go from infinity to zero if the switch is good.

The actual electrical work in this case is possibly easier than getting the tank off. If you end up replacing the switch, have the new one ready to go so you can get the new one in as quickly as possible to minimize brake fluid loss and exposure to the air. It probably wouldn't hurt to bleed the brakes after replacing the switch.
Call me Mel. Some years ago- never mind how long precisely- having little or no money in my purse, and nothing particular to interest me at home, I thought I would ride about a little and see the other parts of the world.

Wobbly
Posts: 247
Joined: Mon Aug 07, 2017 8:48 pm

Re: Front Brake Light Not Coming On

Post by Wobbly » Sat May 26, 2018 9:30 pm

Welcome aboard !

Your test process is well thought out and will tell you what you what to know. Your issue is most likely corrosion on the 2 switch contacts caused by brake fluid, or a bad hydraulic switch.

Since the original electrical system was designed and supplied by Bosch, lots of the switches, regulators, and relays are common to other VW, Mercedes, Volvo, Audi, etc cars. Therefore, replacement parts can often be had from your local auto parts stores.

After replacing the switch (on the very front of the brake master cylinder) I'd highly suggest a complete brake fluid flush with fresh DOT4 brake fluid. The brakes may "feel" OK, but old brake fluid will definitely not allow you to stop as well.
After 20 years as a professional bike mechanic and 30 years as an engineer I know just enough to be dangerous !

fgr33n
Posts: 3
Joined: Sat May 26, 2018 8:49 am

Re: Front Brake Light Not Coming On

Post by fgr33n » Tue May 29, 2018 8:10 am

So it sounds like the first step is to pull the tank and check the switch-- hopefully tonight. I'll post how it goes. Thanks a million for the input!

Wobbly
Posts: 247
Joined: Mon Aug 07, 2017 8:48 pm

Re: Front Brake Light Not Coming On

Post by Wobbly » Fri Jun 01, 2018 5:18 pm

If you want to do a first rate job.... That is to say, if you'd like your bike to stop like a modern disc brake would, that's an inexpensive add-on while you got your brake system taken apart for service.

1) Fit a new set of EBC brake pads. Chances are, yours are the original equipment; old and hard.
2) Replace the OEM flexible rubber brake hose with a modern PTFE lined and steel braided brake hose. There are lots of sources for these... Spiegler (https://spieglerusa.com/) is probably the best known, Bob's BMW also makes them.
3) Then since the bike has sat for several years, there will be internal crud that will break loose once the caliper is heated by braking. Simply take the time to run new brake fluid through the system again after you ride the first 500 miles or so.

Cheers !
After 20 years as a professional bike mechanic and 30 years as an engineer I know just enough to be dangerous !

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