Early master cylinder with warning light switch - any way to use it for brake lights?

[Jonny042]

I've backdated my braking system to manual brakes, but don't have a mechanical brake light switch on my pedal cluster.

The master cylinder I bought included a switch on the side that I assumed I'd be able to use for the brake lights - I've got it all wired up and everthing tested and buttoned up. Today I filled and bled the brakes - no brake lights......

So, I did a little research and find out that switch is actually a brake pressure warning light switch, and is a differential pressure switch designed to alert when one half of the brake system loses pressure.

Can anyone explain to me how that switch works? Not electrically, it's a simple two prong normally open switch, but hydraulically/mechanically? Hoping I might be able to salvage this......

I'm getting near the end of the project and would like to get the car on the ground and the thought of having to re-wire the brakes or figure out mounting a mechanical switch is going to be a huge headache. I've got it all buttoned up and would hate to have to tear into the wiring......

I'm open to suggestions..... thanks in advance!!!!!!

[daddygo]

You probably have to reinstall the manual brake switch on the brake pedal. I always thought the manual switch was a funny design considering VW didn't use it, at least not in later cars. As you figured out, the safety switch will only let your know if pressure is low, which is not what you need to run the brake lights.

[Trackrash]

I wonder if you changed that switch to one that the old bugs used? My '67 Beetle uses a pressure switch on the MC, to operate the brake light, that looks suspiciously like the one on the old 911 MC. Maybe someone knows if that will work?

They are $8. Might be worth trying. https://www.ebay.com/itm/191703842044

[RDM]

Isn't this the way the SC brake lights work? Two identical three-prong pressure sensors, either of which trips the brake lights, and the difference will trip the warning light.

I'm sure you already know that, so obviously I'm not understanding the circumstances.

[Jonny042]

Thanks, guys - yeah, the hydraulic pressure switches that SC's and Carreras, and indeed, VW, used, are what led me to assume the switch would work.

When I went looking for a master cylinder I found the one with a switch, made the (completely incorrect!) assumption it was a brake switch, and went on my way wiring the whole thing up to operate the brake lights.

The stupid (well, the stupidest) part is that the MC came with a single pole switch. I thought "well that's dumb, why would they use a single pole switch, that makes no sense" so then I ordered a not very inexpensive (at $65.00) 2-pole switch.

The more I think about it the more I realize the MC must be designed with a separate passage with a piston suspended between two springs and that somehow triggers the switch. Even if I could fit a pressure switch (the threads are different) I would probably have to gut the assembly and would lose the independent action of the dual circuits.

I could add a t and the $8.00 VW switch to one of the brake lines. A million beetle owners can't be wrong...... and it would save me having to re-wire the switch to the footwell area (my wires are all carefully and obsessively loomed and jacketed and heat shrunk and wire laced, trust me, it's not a trivial matter to do it properly...)

I also wonder if there is a compatible master cylinder that has one or two brake switches built in? Maybe from an earlier car.......

[Jonny042]

I guess I'm thinking out loud, here, but on the other hand....... a mechanical switch on the brake pedal is a much better option, safety wise..... get those lights illuminated if you even THINK about braking, as opposed to the hydraulic type that aren't really that responsive.

Painless wiring has a switch (probably a re-badged dorman or standard part) that looks like it would be easy to adapt, and has an adequate 20 amps rating:

[Trackrash]

Originally Posted by Jonny042

Thanks, guys - yeah, the hydraulic pressure switches that SC's and Carreras, and indeed, VW, used, are what led me to assume the switch would work.

When I went looking for a master cylinder I found the one with a switch, made the (completely incorrect!) assumption it was a brake switch, and went on my way wiring the whole thing up to operate the brake lights.

The stupid (well, the stupidest) part is that the MC came with a single pole switch. I thought "well that's dumb, why would they use a single pole switch, that makes no sense" so then I ordered a not very inexpensive (at $65.00) 2-pole switch.

The more I think about it the more I realize the MC must be designed with a separate passage with a piston suspended between two springs and that somehow triggers the switch. Even if I could fit a pressure switch (the threads are different) I would probably have to gut the assembly and would lose the independent action of the dual circuits.

I could add a t and the $8.00 VW switch to one of the brake lines. A million beetle owners can't be wrong...... and it would save me having to re-wire the switch to the footwell area (my wires are all carefully and obsessively loomed and jacketed and heat shrunk and wire laced, trust me, it's not a trivial matter to do it properly...)

I also wonder if there is a compatible master cylinder that has one or two brake switches built in? Maybe from an earlier car.......

My thinking is that there would be enough pressure, even between the two circuits of the MC, to activate the VW switch.

Now that I think about it, maybe just one of the SC switches could be wired to work?

Wouldn't be that hard to set up a test?

[Jonny042]

Originally Posted by Trackrash

My thinking is that there would be enough pressure, even between the two circuits of the MC, to activate the VW switch.

Now that I think about it, maybe just one of the SC switches could be wired to work?

Wouldn't be that hard to set up a test?

Hey Gord,

The fitting for the switch is different - straight threads (10mm I think) and the switch is sealed with an O-ring, not a tapered thread. So my guess is there isn't full pressure available at that port but somehow there is something mechanical activating the switch - it has a pushrod/plunger in fact.

Anyway I has a look a the pedal cluster and figure it will be easy to build a bracket for a normal switch, and have ordered a nice metal switch as opposed to the plastic one:



Although re-routing the wiring will be a bit of an annoyance, it's not as bad as it could have been. I routed the brake switch wires in their own PVC Jacket (you can just see them hiding behind the fuel tank on the way to the smuggler's box in the pic below) so they'll be easy enough to pull back, I'll open the main harness up and re-lace it, bing bang boom, it'll be done. Minor setback.

[kium]

Originally Posted by Jonny042

I've backdated my braking system to manual brakes

Jonny, can you tell me what size MC you chose and why?

My 2.2T was originaly equiped with 19mm, but now there's a straight forward replacement of 23mm.
I'm wondering if that would be a good upgrade somehow "equivalent" to bigger brakes.

[Jonny042]

I have stock sized calipers , so I went with the standard sized 19mm MC. A larger MC will only increase pedal pressure unless you change to larger pistons in the calipers.

In other words, I doubt it would be a good idea to swap only the MC.

[kium]

Ok, I think I understand, a larger MC increases the volumen of brake fluid shifted but not the pressure on the calipers.

[gomezoneill]

That's one clean looking car. How about some more photo's including the engine compartment?

[RDM]

Originally Posted by kium

Ok, I think I understand, a larger MC increases the volumen of brake fluid shifted but not the pressure on the calipers.

I don't think so. For a given pedal pressure, a larger master cylinder will produce a lower line pressure. The 23mm master cylinder has about 50% more area than the 19mm, giving you about 50% less mechanical advantage.

[911pcars]

Simple answer. A typical MC pressure switch can’t be adjusted. A mechanical switch can, to allow the brake lights to energize at any desired amount of brake pedal movement. You want the lights to go ON with a feather touch? Done. You want them to light up only in a 4-wheel lockup? Done.

Sometimes the best mods aren’t in the “buy it in a bag and bolt on in 7 minutes”.

S

[kium]

Originally Posted by RDM

I don't think so. For a given pedal pressure, a larger master cylinder will produce a lower line pressure. The 23mm master cylinder has about 50% more area than the 19mm, giving you about 50% less mechanical advantage.

Thanks for your explanaition, RDM
I agree, it's more or less the concept I understood, but I didn't explain it well.

[Jonny042]

Originally Posted by gomezoneill

That's one clean looking car. How about some more photo's including the engine compartment?

How much time do you have? LOL. There are 50+ pages of posts and pics at:

Project Heavy Metal - an all steel classic, remastered

Here's the latest. Should be firing up soon..... let's hope!!