Brake lights not working - nor brain! – Solved!!!

Thanks for your vote of confidence in my limited knowledge and abilities, Ron!
The end of the switch has to be conical to form a seal, right?
It also eventually occurred to me, the union that I'm screwing the brake switch into is made of brass, right? Therefore the softer thread in it couldn't mash up a harder steel thread like the switch in question, surely?
I realise that both Demetris and Mark Cobraboy DID try to warn me that new parts are often unsatisfactory.
Anyway, I'll be removing the link I posted earlier in this thread, to the vendor who sold me the shonky part, because they now won't refund me or send me a replacement unless I prove that I am a certified mechanic. Grrrr! I shan't be buying anything from autodoc again.
I didn't mention before just how much fun I had been having on my first failed attempt, forced to use an adjustable wrench because my set of stubby spanners stops at 19mm! There's simply no room to turn it more than a small fraction, requiring much faffing about, turning the too-long-for-the space-available wrench first one way, then the other, all whilst avoiding many and various wires and hoses and hard brake lines.
I was about to leap from my upper window in sheer frustration at the internet singularly failing to serve me a link to a vendor offering a single 22mm open end short spanner. I could only find them as part of sets, which of course would duplicate the spanners I already own. All this after having already toured the various local actual physical tool supply shops, all staffed by youngsters who didn't know what a spanner even is.
At that very moment my next door neighbour, with whom I had been chatting (okay, moaning!) about my car troubles yesterday, rang my door bell and presented me with a nicely modified 22mm open/ring combo spanner, which he had already thoughtfully cut in half, having bevelled and smoothed both edges to prevent injury. Top bloke! My faith in humanity is restored. His tool gift really made my day. Thanks Mark!
I've ordered another replacement switch, from a different supplier this time, and we'll see what happens on my next attempt.
This has become a right old saga!
 
mrtask said:
Thanks for your vote of confidence in my limited knowledge and abilities, Ron!
The end of the switch has to be conical to form a seal, right?
It also eventually occurred to me, the union that I'm screwing the brake switch into is made of brass, right? Therefore the softer thread in it couldn't mash up a harder steel thread like the switch in question, surely?
I realise that both Demetris and Mark Cobraboy DID try to warn me that new parts are often unsatisfactory.
Anyway, I'll be removing the link I posted earlier in this thread, to the vendor who sold me the shonky part, because they now won't refund me or send me a replacement unless I prove that I am a certified mechanic. Grrrr! I shan't be buying anything from autodoc again.
I didn't mention before just how much fun I had been having on my first failed attempt, forced to use an adjustable wrench because my set of stubby spanners stops at 19mm! There's simply no room to turn it more than a small fraction, requiring much faffing about, turning the too-long-for-the space-available wrench first one way, then the other, all whilst avoiding many and various wires and hoses and hard brake lines.
I was about to leap from my upper window in sheer frustration at the internet singularly failing to serve me a link to a vendor offering a single 22mm open end short spanner. I could only find them as part of sets, which of course would duplicate the spanners I already own. All this after having already toured the various local actual physical tool supply shops, all staffed by youngsters who didn't know what a spanner even is.
At that very moment my next door neighbour, with whom I had been chatting (okay, moaning!) about my car troubles yesterday, rang my door bell and presented me with a nicely modified 22mm open/ring combo spanner, which he had already thoughtfully cut in half, having bevelled and smoothed both edges to prevent injury. Top bloke! My faith in humanity is restored. His tool gift really made my day. Thanks Mark!
I've ordered another replacement switch, from a different supplier this time, and we'll see what happens on my next attempt.
This has become a right old saga!
Click to expand...

The end of the switch has to be conical to form a seal, right?

IIRC the switch is a taper thread with a flat end, be aware that screwing the switch in all the way can actually split the brass T, this will not be apparent until pressure is applied, with the resultant loss of fluid. I know this because only three days ago I bled up a new build car and the BL switch was leaking at the T, and the T was cracked !

I usually wrap a small amount of PTFE tape around the threads and do up until firmly fitted, but not over tight.
 
@cobraboy So it is the copper or brass washer that forms the seal then? So the fluid can perhaps creep down the thread, but shouldn't get past the sealing washer? What is the reason for using the PTFE tape then? Is it the case that if the correct washer is fitted, the switch won't 'bottom out' in the brass union, but if no washer is used the switch can cause a crack in the union? Sorry for all the questions!
 
The one I recently had to change was a taper thread, the thread forms the seal by getting gradually larger, helped by a little tape.
If you have a parallel thread then yes a copper washer would be good. Either way there is no conical seat on the switch as your photo indicates.
A parallel thread won't crack the T if over tightened, but may leak without a copper washer.
 
@cobraboy Thanks for your patient explanation.
Slowly, the lights are coming on here. Not the actual brake lights on my car, but the dim bulb up in my skull!
Sorry, but may I pick your brain some more, please?
Situation update – I received the next replacement switch in the post this morning, and tried once more to install it.
This time, on removing it I first compared the defunct switch to the new part, and what d'ya know?! NOT the same.
I had (again) mistakenly ordered the wrong thing. Blast!
My defunct switch has a tapered thread. Until I read your helpful contributions, Mark, I hadn't even known that was "a thing", but comparing the two it was quite apparent, with the benefit of my newly acquired knowledge!
So, what size do I need!?!? The defunct switch is bigger at the tip than the 3/8 UNF parallel thread switch(es) I had mistakenly been trying to fit, and keeps getting bigger still as it is a tapered thread.
I measure it as being 0.4"... The internet tells me 0.43" would be 7/16...
So the next size up would seem to be 7/16 UNF tapered thread, but I can't find a switch with that size listed anywhere on the internet. There are listings aplenty for banjo bolts and unions, but not BL switches.
I put the defunct switch straight back in, mindful of not wanting air in the system, before realising I should've maybe taken a photo or two to help identify the correct part. Doh!
Man oh man, I thought this was going to be so easy! Still, at least on my second attempt I actually had the right spanner for the job!
Suggestions and advice humbly solicited!
 
I think maybe you got sent Metric M10 x 1.0 switches which would explain why it cross threaded.

I just went to my brake bits tub. A metric M10 x 1.0 male fitting will not screw into a 3/8 x 24 UNF female fitting, it cross threads approx 1 turn in - like your switches !

The plot thickens....................

It is possible that if you have a taper thread on the original switch, that the thread is 1/8" NPT ( National Pipe Thread )
It may be that the port on the brass fitting has that one hole tapped 1/8" NPT Whereas all the others are 3/8" UNF.

1659110556198.png

1/8" NPTF switch here Genuine Lucas Brake Light Switch - SMB423 | eBay

I learnt something today

NPTF thread roots are designed to interfere with crest of the mating thread, which creates a mechanical seal through thread form deformation at assembly, while NPT thread roots are designed to allow clearance with mating thread crests on assembly.

If you got a nice piece of hardwood and drilled a series of holes in it you could make a super display stand for all your switches .........................
 
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I just get more and more confuddled!
I could've sworn that Colnerov commented here earlier, and that I replied to him, but his contribution seems to have vanished, and my reply was apparently still just a 'draft' and hadn't been posted. Most peculiar. Hey-ho.
@cobraboy Thanks for taking even more time to first rummage in your brake bits'n'bobs assortment, then venture more suggestions. I appreciate it.
Helpful little diagram that, thanks.
Sooo, and with apologies for obviously being pretty damn dim, a 1/8" NPT switch is smaller than 3/8" x 24 UNF at its tip, but as near as damn it the same size at the end of the thread? That'd also be too small then. The two switches I've had so far didn't come with washers, so I was going to re-use the copper washer off my defunct switch, but the I.D. was a fair bit bigger than the threads on the new ones....
Now I'm leaning towards it being 1/4" BSP switch that I need, and hopefully loosening the duff one and measuring the O.D. will confirm it...
I should probably buy myself a Vernier gauge, while I'm out shopping for a multimeter...
 
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Hi, Yes, apologies. I did post that I thought it was a metric threaded switch. I then started to doubt myself and went out to the workshop to check. However I then remembered that Land Rover used 7/16" thread unions in some locations and was checking those as well and ended up confusing myself so I removed the post so I didn't make matters any worse. It looks like Cobraboy might have sussed it. I suppose you could try taking it out again and screw a 3/8" bolt in to see if that fits, has a parallel or tapered thread and that the thread in the 'T' piece is not compromised. You could then check that a 3/8" nut goes on or not the collection of switches.

Colin
 
@colnerov No worries! No need to apologise! Glad I'm not going mad.
I think I might just have sussed it now, thanks to this helpful page: Hydraulic fitting thread chart
The section on NPTF (National Pipe Tapered Fuel) states “The NFPA (National Fluid Power Association) does not recommend the connection for hydraulic applications" – hmmm, okay then –
so then I looked at BSPT (British Standard Pipe Tapered), searched some more, and found 1/4" BSPT brake light switch. That's gotta be it!
22mm wrench size, and visible in the photo is the same mysterious number engraved on one of the flats as on my defunct switch.
As to why my car has a union that takes that size, and not what my Parts Catalogue (and Demetris) list, that's still a quandary.
I had no idea there were so many thread standards. This has been educational, if frustrating!
Why don't the manufacturers engrave or print the size on the flats of the switch? Why isn't the copper washer supplied with a new switch? I have so many questions!
 
The genuine Lucas switch does not have a tapered thread. If it did have a tapered thread, there would be no need for a copper washer. The genuine switch screws right in with the copper washer tightened against the body of the switch. There is no need for thread tape or anything else, indeed for brake connections, there should never be any need for tape. I have a NOS item, I will take a look tomorrow and post some pics.

Ron.
 
As promised here are some photos of a NOS Lucas brake light switch. The thread is clearly not tapered featuring 24 threads per inch (TPI). The seal is formed at the tip in the same way that a brake hose is sealed. As an example, I have also included two photos of a NOS Girling brake hose for the Rover, note the thread and tip.

Ron.
20220803_100630.jpg
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It is my belief that those two items pictured do not seal at the tip, the end of the fittings simply do not have enough of a conical seating area to form a seal. I believe both those items use a copper washer to form a seal, as stated by you in your post # 32 above.
 
I just pulled one from a sixties Jag. It has taper thread, no washer.
It’s yours if you want it FOC.
The last brake fitting seals at the tip. 343A7644-B291-4FCC-933F-1C2F8CC3C5D4.jpegA0D9AC04-8D0C-4F5D-8BE6-EEA1DA4F7C18.jpegAE48D131-2DC1-4F67-B9CF-F611201D2B1D.jpegDCD12045-B1F9-403B-AEE4-48BF69346404.jpeg
 
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Thanks for your contributions and offers of good used parts, Ron, Mark.
The answer to my conundrum turned out to be – waaait for it – 1/8" BSP: part number STC1689 from Dingocroft Land Rover parts.
I thought I had ordered 1/4" BSP, and that's what it says on their webpage, but they sent 1/8" and it fit!
Third time was the charm!
 
Good morning all,

I’ve been reading this thread hoping to figure out where I can find this brake switch :hmm:, apparently it’s a hydraulic switch controlled by brake fluid pressure?

The brake lights on my P6 stopped working as well, and this can become dangerous.

Please give me a hint.

Thanks,

Wilfred
 
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