Brake light appears under heavy braking ??

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Brake light appears under heavy braking ??

Postby simonbuk » Sun 04.09.2016, 20:58

Just been out for a while and noticed under heavy braking that the brake light appeared to come on. At least I think it was the brake light, it flashed for a second or so then disappeared.

Is this normal/known ?!
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Re: Brake light appears under heavy braking ??

Postby Rambo » Sun 04.09.2016, 21:21

Yes. First check your brake fluid levels in the reservoir. You will probably find they are slightly low and need topped up and all will be OK. But also check all brake lines in case there is a small leak of hydraulic brake fluid

This happened to me once under heavy breaking. I topped up with the relevant DOT brake fluid and all was ok

However, if it's been a couple of years since a brake fluid change was done on your car, renew with new hydraulic fluid as it is hygroscopic ie absorbs water which has a different compressibility to oil
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Re: Brake light appears under heavy braking ??

Postby simonbuk » Sun 04.09.2016, 21:28

Ok cheers, I'll check the fluid level when daylight returns.............
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Re: Brake light appears under heavy braking ??

Postby Giniw » Sun 04.09.2016, 22:39

I agree with Rambo.
Mine was far from the minimum mark but still it was enough to switch on the tell-tale under heavy breaking.
And I agree that it could be a good opportunity to bleed the system with new brake fluid — it's easier than a normal bleeding since there is probably no air in your system, it's just a matter of pushing the old DOT4 with the new one.
Last edited by Giniw on Mon 05.09.2016, 18:46, edited 1 time in total.
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Re: Brake light appears under heavy braking ??

Postby lotusflasherman » Mon 05.09.2016, 03:09

My SE that's on SORN used to flash the fluid warning light at me under braking, whatever the fluid level. Early brake master cylinders (bmc) with fluid warning had a float switch so I thought 'the float' might have fluid in it and be sinking but acquired a spare bmc to fit and found the fluid sensor is actually a printed circuit board that sits beneath the black plastic cap on top of the bmc. Bit tricky but it's possible to get it out with bmc still in place so have just swapped the pcb. (Seems easy to get pcb out but tricky to get correct angle to get the replacement in - long and thin and fragile!)

Only did this swap recently so haven't driven the car yet to check the replacement pcb, or got round to dunking the troublesome one in brake fluid to see whats happening electrically with the 'fluid switch'. I've been wondering if they start playing up if brake fluid, which is hygroscopic, has absorbed moisture so was going to try it in new stuff and some old stuff in a jar I bleed brakes into to see if there was any difference electrically. Just curious so I need to investigate ...

I'll update here when I've done it.
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Re: Brake light appears under heavy braking ??

Postby chrisP » Mon 05.09.2016, 10:55

I used to have similar problems, the fluid level was way above the minimum but the light came on. Now the reservoir is filled to the top. I would check for leaks first and then give it a top up. (remember to take some fluid out prior to a pad change though, as pushing back the pistons could cause it to overflow.)

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Re: Brake light appears under heavy braking ??

Postby lotusflasherman » Mon 05.09.2016, 17:47

My light flashed when the BMC was completely full.

Just been investigating the fluid switch on the pcb that I removed and found it's become a vibration sensor not a fluid level warning switch !!!

We are not allowed to post mp4 files here so I put a short vid on youtube ...https://youtu.be/GVJGlWSIZyo

Light should be on as pcb is in fresh air but flashes if I tap it... It begs the question 'How many really still work correctly ?" - are we living in a Fool's Paradise? The Fluid Warning light check uses the handbrake contacts through a diode not the fluid switch. The easy way to check will be when bleeding the brakes let the fluid get low and see if a fluid warning light comes up when the handbrake is off. Meanwhile I suggest a visual check might be more reliable .. :lol:
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Re: Brake light appears under heavy braking ??

Postby Motocompo » Fri 09.08.2019, 07:42

Where can I get this sensor from?

I have the problem that after 3 sec of braking the warning light goes on (95 S2).

After I release the brake pedal the light goes off.

I have new brake pads, the fluid level is a little below max and the brakes work fine.
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Re: Brake light appears under heavy braking ??

Postby Rambo » Fri 09.08.2019, 08:28

I would top up the brake fluid level first a d see if the problem goes away. It can be particularly noticeable going down a slope if the reservoir level is just a bit low
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Re: Brake light appears under heavy braking ??

Postby Brit-Car-Nut » Fri 09.08.2019, 14:00

The sensor is part of the reservoir, E100J0009F so not an independent part. The reservoir went obsolete in 2016. SJ still shows stock at £20.27 each.
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Re: Brake light appears under heavy braking ??

Postby Saltire » Fri 09.08.2019, 14:19

Mine used to do this too, Simon - as did ultimatt’s. When I did my brake refurb last autumn, I replaced the reservoir (same master cyclinder), and changed the brake fluid, and I haven’t had the problem since.
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Re: Brake light appears under heavy braking ??

Postby rip » Fri 09.08.2019, 14:44

Motocompo wrote:Where can I get this sensor from?

I have the problem that after 3 sec of braking the warning light goes on (95 S2).

After I release the brake pedal the light goes off.

I have new brake pads, the fluid level is a little below max and the brakes work fine.


That is almost certainly the problem. The sensor is set to warn you of even a slight drop in level.
Mine was like that when I bought it. The previous owner thought it was just a quirk of the car.
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Re: Brake light appears under heavy braking ??

Postby Motocompo » Sun 25.08.2019, 13:46

I topped the brake fluid and the problem is solved. No more red light.

Also thanks for the hint with the reservoir and where to purchase it.

Special thanks to all! YOU ARE GREAT!!!
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Re: Brake light appears under heavy braking ??

Postby Rambo » Sun 25.08.2019, 16:16

Rambo wrote:I would top up the brake fluid level first a d see if the problem goes away. It can be particularly noticeable going down a slope if the reservoir level is just a bit low


More brownie pints :drunk:
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Re: Brake light appears under heavy braking ??

Postby Rocklobster » Fri 27.09.2019, 20:46

This really isn't a problem. The reservoir for the master cylinder is positioned such that when you brake reasonably hard or for a reasonable duration, the fluid sloshes towards the front, whereas the level sensor is at the rear of the reservoir, therefore the sensor is uncovered for a short duration and your brake warning light flashes. There's nothing to be concerned about as long as your fluid level is ok.

That doesn't mean you need to keep topping it up, just make sure it's full when the brake pads are new and as long as you don't have a leak, the system should be ok. If the fluid drops to the lower level mark, that means either that you have a leak, or that you should check your brake pads for wear.
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