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Hydraulic Brake Light Switch
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Rootes75



Joined: 30 Apr 2013
Posts: 2744
Location: The Somerset Levels

PostPosted: Mon Nov 19, 2018 11:23 am    Post subject: Hydraulic Brake Light Switch Reply with quote

I have connected up my hydraulic brake switch on my Commer lorry and I get no life from it at all.
I know fluid is passing into it and it isn't blocked and the wiring is new and to a terminal that I know is live.
Is there anyway to check the switch, or looking round on ebay is it just easier to replace it?
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1947 Hillman Minx
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ukdave2002



Joined: 23 Nov 2007
Posts: 3470
Location: South Cheshire

PostPosted: Mon Nov 19, 2018 8:14 pm    Post subject: Reply with quote

There have been lots of issues with some brands of this switch, when my dad had his MM he had about 6 that failed either straight out of the box, or within a few months. It was so bad that he rigged up an additional dashboard warning lamp wired to the brake lamps Shocked

To check the switch simply measure the resistance when the system is pressured.

Dave
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ukdave2002



Joined: 23 Nov 2007
Posts: 3470
Location: South Cheshire

PostPosted: Mon Nov 19, 2018 8:19 pm    Post subject: Reply with quote

There have been lots of issues with some brands of this switch, when my dad had his MM he had about 6 that failed either straight out of the box, or within a few months. It was so bad that he rigged up an additional dashboard warning lamp wired to the brake lamps Shocked

Dave
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jp928



Joined: 07 Jun 2016
Posts: 247
Location: Australia

PostPosted: Wed Nov 21, 2018 4:11 am    Post subject: Reply with quote

One car I had many years ago had an indicator for working brake lights - round can like a flasher, connected to brake circuit; mounted in a plastic case that screwed to bottom edge of dash, lit a bulb behind a plate saying 'Brakes', accompanied by an audible click. These days you would run a lead from switch output to a 12v LED (via a resistor) mounted in some sort of bezel.
jp 26 Rover 9
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Phil - Nottingham



Joined: 01 Jan 2008
Posts: 1254
Location: Nottingham

PostPosted: Wed Nov 21, 2018 11:32 pm    Post subject: Reply with quote

I have one of those accessory warning lights on 3 of our classics as it warns of a single bulb failure.

The hydraulic switches can be very insensitive and only work when brakes are full on when they age and new ones are very inconsistent and can even leak after a short time
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lowdrag



Joined: 10 Apr 2009
Posts: 1250
Location: Le Mans

PostPosted: Thu Nov 22, 2018 2:12 pm    Post subject: Reply with quote

I've no idea if you have managed to repair your switch, but I always carry a spare like this in my E-type. Not dear, and I've replaced only two over 30 years.

https://www.sngbarratt.com/uk/#!/English/parts/9814f8c3-58d9-4a86-a60b-401be3902ece

https://www.sngbarratt.com/uk/#!/English(US)/parts/74c68473-49f8-481d-b474-c1ab92e15360

Hope this helps.
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Peter_L



Joined: 10 Apr 2008
Posts: 2270
Location: New Brunswick. Canada.

PostPosted: Thu Nov 22, 2018 2:47 pm    Post subject: Reply with quote

I never had a problem with any that I had, but years back I remember a mechanic saying that depending on location it is possible for an air bubble to get trapped and stay there. I guess it's possible, but in theory seems more likely than in practice. Opinions welcome because this idea was floated to me over 50 years ago.
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Phil - Nottingham



Joined: 01 Jan 2008
Posts: 1254
Location: Nottingham

PostPosted: Thu Nov 22, 2018 6:55 pm    Post subject: Reply with quote

I have that and it could well be true but loosening a failing switch and pressing the pedal down to eject some fluid, I had only last week had no effect. A old battered one I put in worked as good as a mechanical switch
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Rootes75



Joined: 30 Apr 2013
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Location: The Somerset Levels

PostPosted: Thu Nov 22, 2018 8:53 pm    Post subject: Reply with quote

Thats strange with SNG comment, looking through the original parts list the lockheed switch is also used on Jaguars.
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ukdave2002



Joined: 23 Nov 2007
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Location: South Cheshire

PostPosted: Fri Nov 23, 2018 11:27 am    Post subject: Reply with quote

In reality the only difference between the switches will be the thread size; they all should operate from around 50PSI (light braking) to over 2000PSI. And be capable of switching the current a couple of brake bulbs draw.

The symptoms that I saw with modern reproduction Lucas switches were either requiring a very high pressure to operate or failed "OC" The only failure I ever had with an original switch was a "SC" fail....possibly although the modern and original switches look the same externally, they are engineered differently internally?

peterwpg wrote:
I never had a problem with any that I had, but years back I remember a mechanic saying that depending on location it is possible for an air bubble to get trapped and stay there. I guess it's possible, but in theory seems more likely than in practice. Opinions welcome because this idea was floated to me over 50 years ago.
A bubble in the system can't affect local system pressure, and said pressure has to be the same on both "sides" of the bubble.

Dave
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