First, have someone push the brake while you have a continuity tester on the switch terminals on the master cylinder. These pressure switches are FAMOUS for failure. I carry an extra in my toolbag in the car if that gives you any indication. They can often be swapped out with almost zero brake fluid loss and no bleeding necessary.
Pro tip: Install two switches, and make the wires long enough to reach both, so when it fails you merely move the wires. And it will fail.
If the switch is good then you'll have to figure out why no juice. Check for power INTO the switch. It's basic wiring troubleshooting.
The VW dual circuit master cylinders have two brake switches in order to use the warning light that is on the speedo. They use 3-prong switches and are wired to light the warning lamp if one of the master cylinder circuits fail. The wiring is a little complicated, but actually a simple electrical circuit that sends power to the speedo warning lamp if one master cylinder hydraulic circuit fails.
However, as @DannyP pointed out, the switches fail frequently. Much more often than the master cylinder in my experience!
To address both @Michael McKelvey and @James, as you're both talking about different aspects of these switches, Yes, you can wire both of them to light a dash lamp if one of the brake fluid circuits fails. As James said, the wiring is a bit tricky but not too much so (The Bentley Service Manual wiring diagram for a 1970 Beetle shows how they work), but I have yet to see a Speedster/Spyder wired up that way with a working dash light.
OTOH, it makes sense to wire both switches in parallel (slightly different wiring) so that when one fails (it certainly will) the other one will just keep working until IT fails, too. But then it's important to swap out BOTH switches if you lose your brake lights because both switches will be bad at that point. Info I get from the Hot Rod crowd is that these Asian hydraulic switches seem to have a life of around 18 - 24 months, which really sucks for something like that.
Or you could just upgrade to a mechanical switch in the cockpit and put this all behind you.
BTW, Mike - I remember (I think) that you had tried some LED lights from "SuperBright". I just got a pair of 921 LED bulbs for my 3'rd brake light from them (arrived today) and HOLY CARP! Are those things ever BRIGHT! Practically light up the whole shop! Another satisfied customer......
My version requires fabricating a simple mount (I think the mount is included in Mike’s version) and making a bend in the switch arm to allow the lever to ride along the brake pedal arm to actuate it. This also lets you tune in the amount of pedal movement needed for the lights to come on. I also slotted the carpet for the switch arm but just let the carpet flop over the switch to cover it up.
@Gordon Nichols I am pretty sure all Becks are wired that way, at least the Speedsters are. Maybe Spyders too.
I have had one switch failure in 15 years, so it might have something to do with faithfully changing the brake fluid every two years, like you're supposed to. The failure was in 2012 or 2014 or so down at Cory Drake's house.
This was when Bruce sold his white CMC and bought that silver Vintage. I helped him get the carbs/engine running better and visited the Drakes as well as the Stumpps.
I had to detach the pedal cluster to attach the switch. When I reattached the cluster I pinched the wires from the switch. That caused electrical issues until I figured out what I had done. At first, I thought the switch was faulty. The seller sent me another one at no charge. So, I would say his support is good.
I got to the point of checking the juice to the MC and didn't get any reading. I rigged up a screen door cylinder thing and propped it between my seat and the brake peda l(I use it to bleed brakes also) and turned the ignition on and I didn't get any reading at the MC. I pulled all my fuses and checked them and they were ok ( have one empty slot) ? and everything was live when I had the pedal depressed. What next?
Did you try connecting the wires at the switch (bypassing the switch) to see if the lights work?
If they don't, trace the hot wire back to its short. I sometimes have seen fuses that look good but aren't. If it's not at the fuse, try the back of the ignition switch, etc. Once you see where the wire from the MC goes it should be pretty easy to see where it's NG.
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