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Hydraulic Brake Switch Internals

jman15

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After recently replacing this switch on my BJ8 several times decided to cut one up and see what was inside. Attaching several pictures so you can see the results. Basically this is a very simple device where brake fluid applies pressure to a flexible diaphragm that then pushes a copper disc against several metal contacts closing the brake circuit. Several internal springs hold the disc in place. I originally thought brake fluid might be corroding the contacts but I saw no evidence of this and since I have been using silicone brake fluid I didn't think this likely. Looking closely at the contacts I noticed there was a small amount of hard plastic partially covering one of the contact points. My best guess therefore is this could be the primary cause of early failure. These switches may work when first installed but over time will fail when current draw burns the contact points much like our distributor points. And if one or more of these contact points are partially covered by plastic then they will fail much earlier due to the smaller surface area carrying all the current. This would appear to be a manufacturing defect and not something that a simple initial test would uncover prior to shipment. Although this is a Moss part it is manufactured by Intermotor so please don't interpret this post as a rap on Moss. I've always had good luck dealing with Moss and appreciate the service and support they provide our community.

Replacing these switches is a very easy process. If it wasn't for the fact that brake lights are a critical safety item and the switch failure can occur suddenly and unpredictably I wouldn't give it much thought. I know some of you install inside switches mounted over the brake pedal but was looking for something to keep car original. I did look for alternative switches and was surprised how common they are, particularly as a motorcycle part. But for now will see how my new one works and maybe start replacing it as a normal annual maintenance item.

Hope you found this interesting.

Jim
 

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Bob_Spidell

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Neat! Thanks for posting this.

I'd like to know what the actual failure rate is, say, as failures per miles driven (or years?). How many here have had one fail? I've had, IIRC, two in my BJ8; one in the car when I bought it with 64K miles, and I changed it somewhere down the road (not literally*). I don't think my original one had failed, but I changed it out when I was redoing some of the hydraulic lines anyway (and that job is a bit of a bear). Somewhat related, there is a similar type of switch found in some Ford Rangers of certain years--unfortunately, I have one of them--that disengage the autopilot, err, cruise control, when brakes are applied that can allow brake fluid to leak into the electrical section and ignite (even when the truck is parked and not running). Supposedly, there has been some loss of property and IIRC some fatalities (from fire). Whatever else you think about it, silicone brake fluid has the benefit of not being flammable.

Begs the question: Are newer electrical components less reliable across-the-board? My gut feeling is a conditional 'yes.' I've worked in and around the electronics industry and know there is a constant drive to 'reduce the BOM**.' If you're selling ten million units a year of something, and you're an engineer--or accountant--who finds an equivalent component that is ten cents cheaper, well, you get to be a hero for a day. Even if the replacement component fails in actual use, you aren't blamed because 'it met the specs' (I don't think the people responsible for the GM ignition switch fiasco have ever been punished in any way; but I may be wrong). I also just replaced my BJ8's OD solenoid as a matter of course, even though the original had served faithfully for almost 200K miles, and now I'm hearing reports of premature failures in these units. Guess I'll have to carry the old one to keep the new one 'in line.'

* Am I the only one who's getting annoyed that that word 'literally' is getting used, well, literally too much (it seems to have invaded Millenials' lexicon).

** Bill Of Materials - sum of the cost of all components in a device per unit
 

Healey Nut

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Bob_Spidell;1083432 * Am I the only one who's getting annoyed that that word 'literally' is getting used said:
Not nearly as annoying as sustainable, sustainability, challenges , going forward and the very next person who “reaches out” is going to get a slap upside the head ...
Why does everyone talk like a corporate robot???
 

Keoke

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Well I can not vouch for my repair method.
But years ago when it failed I got Teed OFF as I was headed to a California Healey week in frustration I threw the switch up against my brick wall as it bounced around on the ground I panicked realizing I did not have a spare.
Hhowever, with no other choice available I replaced the switch it been working every since N never failed again I do have a spare though--LOL
 

John Turney

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.... Whatever else you think about it, silicone brake fluid has the benefit of not being flammable.

...
I found that silicone brake fluid does burn when exposed to exhaust gases.

When I modified my brakes to dual circuit, I had a leftover port on the 5-way coupling. I added a second pressure switch in parallel to plug the opening. It reduces the current to approximately half on each switch. So far, after 18 years, at least one is still working.
 

Bob Claffie

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The people that talk like that are the same generation for youngsters whose favorite words used to be "like" and "you know" but they've grown up a little !
 
D

Deleted member 21878

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when i had to replace the brake light switch on my TD, i was advised to buy one from Ron Francis. they are a bit more expensive but everyone on that forum thought they held up better.
 

rossco

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Look. (That one rings my bell. Because, look, I know all about it, and I'm an expert, and look, I want to get your attention, and look,don't interrupt me,and don't you forget it.) Politicians and TV pundits are the worst.
 

andrea

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when i had to replace the brake light switch on my TD, i was advised to buy one from Ron Francis. they are a bit more expensive but everyone on that forum thought they held up better.
I also have a TD with brake pressure switch -it works (more than 10 years)
I remember one article ABrake_Light_Relay-002.jpgthat inserting a simple relay avoid the switch contacts hard works (I haven't follow this suggestion but Relay are ready in my drawer)
My AH have the Brake pressure switch and I haven't any problem (5 years from return on the road again)
during the restoration I seriously considered the mechanical switch solution
my TC 1946 have a mechanical switch, it works, and appear original more than 70 years oldest
 

RAC68

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Hi All,

Adding a relay would help if the issue was point burn due high amperage draw from the light circuit. However, adding a relay would not add to switch longevity if the failure was the need for increasing peddle pressure due to a hardening diaphragm as my switch seemed to be.

I modulate brake peddle pressure with regard to how the car was stopping and always assumed the brake lights were on through the full term of this action. My pressure brake switch failure was not that the switch didn't switch but it required increasing peddle pressure to activate the braking indication lights. Although this condition could be the result of contact deterioration, my impression is that this issue is more likely caused by a hardening diaphragm. Additionally, as a user of DOT 5 (Silicon) brake fluid during the life of my 2nd pressure switch, but DOT 3/4 on the 1st, I don't believe the type of fluid used is a contributing factor.

Although my pressure switch still remains in place and wired in parallel to the added electrical switch (as previously mentioned), retaining the original wired switch was more a function of my overall "Change" objective to easily return to concourse rather then retaining as a backup.

Thoughts,
Ray(64BJ8P1)
 

Michael Oritt

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Ray--

You said "I modulate brake pedal pressure with regard to how the car was stopping and always assumed the brake lights were on through the full term of this action. My pressure brake switch failure was not that the switch didn't switch but it required increasing peddle pressure to activate the braking indication lights."

Not to get dramatic but that is the insidious nature of these switches' failure mode--when one stomps on the brakes the circuit is made and lights go on but with moderate pressure the switch sometimes does not close and there are no brake lights.

As part of tech inspection prior to a race the inspector commonly stands behind the car and tells the driver to hit the brakes. People usually stand on the pedal and the brake lights light up, but it is not uncommon to come up on a car approaching a corner whose brake lights do not work, as the driver is modulating pressure to keep the brakes from locking up. And yet if I tell the driver that his brake lights are not working or are doing so intermittently he stands on the pedal and insists I must be wrong.

I think these switches are maintenance items and perhaps they should be replaced annually. It's not very difficult or expensive to do so.
 
OP
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jman15

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Hi Ray,

Read your response and it got me thinking about the randomness of the switch not working unless one presses hard on the brake. Would suggest there are three possible reasons for this where one or more may apply. 1) hardening of the diaphragm. When I took the switch apart the diaphragm seemed to be quite flexible but then again I never saw it when it was new. A slightly hardening diaphragm could be a likely cause as it would take additional pressure to overcome its resistance. 2) corrosion of the contact points. Again additional pressure might allow contact to complete. 3) hard plastic deposit on one or more of the contacts. Again this would be a manufacturing defect caused by the molding of the part. In this case a small amount of plastic on the side such as I found could be the cause. I didn't show it in my photos but the copper disc is free to move slightly in the slot. So when pressed the disc moves toward the contacts and if it encounters the plastic it could tilt enough to still make contact with both points. When new and if neither of the first two reasons apply then it would work fine. Over time though it would begin to fail when the brake pedal is only lightly pushed. In the end I think your idea of parallel switches or as Michael suggests treating this as a maintenance item is the solution until such time as a better part is available.

Jim
 

John Turney

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Do you mean SBF bursts into flames when exposed to exhaust gases? Sounds like a serious safety issue, which I've not heard elsewhere. The flash point is greater than 400degF which, I suppose is conceivable (at least near the engine):

https://www.clearcoproducts.com/pdf/msds/specialty/MSDS-DOT-5-Silicone-Brake-Fluid.pdf
Yes, I have Kirk headers. They have a slip joint in one branch. I dripped some DOT 5 on that joint from a poor MC discharge fitting and it caught fire. Fortunately there wasn’t too much damage.
 
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The primary brake light switch, as originally fitted, hasn't been changed in the 40 yrs I've owned the car. I did add a second pressure switch, strictly for the 3rd brake light, so we'll have to wait and see if the reproduced item fails before the original.

As far as over used phrases go, how about "clinically proven"? I've heard that BS-line on so many commercials that it has become meaningless.
 

Bob_Spidell

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...
As far as over used phrases go, how about "clinically proven"? I've heard that BS-line on so many commercials that it has become meaningless.

... and irritating. If you can read the fine print--they don't give you much chance on TV--it's always 'proven' by trials done exclusively by the manufacturers of the product (who are obviously unbiased /s).
 

bob hughes

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I have experienced problems with my brake lights and at the time I pinned it down to the pressure switch.

I installed a new one and all was ok for a few weeks and everything went south again - I then installed the old one just for fun and it worked!! I have since found that the connection into the relay can be the culprit so I go there first.

I have also noticed that you have to apply some pressure to the brake pedal before the lights come on I am not sure that this is correct as some times I just touch the pedal to slow down a tad - and I do not think that the lights come on then. My daily driver is quite responsive on the brake lights.

:cheers:

Bob
 
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