Brake warning light

[nick]

My brake warning light stays on until I disconnect the wire from the master cylinder sensor. Is there a fix for this or does it require a new sensor? The reservoir is topped up. I recently changed fluid while installing stainless braided lines. Looks like it doesn't like clean brake fluid.


nick
'79 TR7

[darrellw]

Hi Nick,

All the sensor does is tell you if there is a pressure difference between the front and rear circuits. If you are getting pressure/flow from both circuits, then either the sensor switch is sticking, or the shuttle between the two circuits (that depresses the switch) is stuck.

-Darrell

Darrell Walker
66 TR4A IRS-SC CTC67956L
81 TR8 SATPZ458XBA406206
Vancouver, WA, USA

[jclay (RIP 2018)]

Is your hand brake on? Those both use the same light.

Clay

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[nick]

I just realized that the first step in the Leyland repair manual when bleeding is to disconnect and remove the pressure switch. Anyone know the reason for that? Would skipping that step have anything to do with my problem?

The hand brake is off. If it were on the light would stay on even if I disconnected the pressure switch.


nick
'79 TR7

[jclay (RIP 2018)]

Yes, There is a piston in the master cylinder that will shuttle when one side of the braking system looses pressure, amusing that there is a pressure leak and blocks that side to keep from loosing more fluid. The pin on the pressure warning switch will fit into the a grove in the piston, activating the warning light. The piston will not move back into position until the warning light switch is removed.

When you bleed the brakes, you are essentially causing one side of the system to loose pressure.

Clay

[url="http://web.me.com/jclaythompson/Site/Welcome.html"]My Triumph Site[/url], [url="http://web.me.com/jclaythompson/Technical"]Technical Stuff[/url], [url="https://public.me.com/jclaythompson"]My Public Folder[/url], [url="http://web.me.com/jclaythompson/tr8/Welcome.html"]My TR8 Site[/url]

[FI Spyder]

The switch needle rides on a high point on a cam (ridge). If it falls off due to pressure differential between front and rear (happens when you bleed brakes) it won't reset as the wall is too steep for it to climb up and in fact prevents the piston from resetting. If you remove switch the piston is free to reposition itself when you step on the brakes while it is out. You should make sure the piston is free to reposition. When I rebuilt my master cylinder when I first bought the car (just because I didn't know when it was last done) the differential piston was frozen in place and required some levering with a screw driver to get it out and clean it up.

Do not leave the brake light on (switch activated). I did, waiting until I got home (when it activated on the mainland) and it burnt the switch out requiring a new switch.



- - - - TR7 Spider - - - - - - - - 1978 Spitfire - - - - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - - Yellow TCT

[nick]

All good stuff. Thanks for all that info. I assume that when the switch is removed fluid does not escape. Correct?

I've gotten myself pretty well educated on the engine with two rebuilds in the past year, but, except for a brake pad change and switching to the stainless braided lines, I'm not too smart on the braking system.


nick
'79 TR7

[darrellw]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by nick</i>

All good stuff. Thanks for all that info. I assume that when the switch is removed fluid does not escape. Correct?
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

I shouldn't, unless you have some internal leakage in the master (which is possible). So be prepared. Also, the plastic switch may not come out easily, and may in fact break off, so take some care.

Darrell Walker
66 TR4A IRS-SC CTC67956L
81 TR8 SATPZ458XBA406206
Vancouver, WA, USA

[nick]

Problem solved. Removed the switch without any loss of fluid. Stepped on the brake pedal a few times. Reinstalled the switch. No light. Thanks for the help.


nick
'79 TR7

[darrellw]

I love it when the fix is easy!

Darrell Walker
66 TR4A IRS-SC CTC67956L
81 TR8 SATPZ458XBA406206
Vancouver, WA, USA

[UKPhilTR7]

Break warning light? Am I missing something, my car has not got one, is that because it is a UK model?

[saabfast]

UK cars have a brake warning light. It is the same one as for the handbrake, goes out when the h/b is released but comes back if the master cylinder switch detects a problem as described above.

Alan
Saab 9000 Stg 1
Saab 9000 2.3 FPT Auto
Saab 2.3 LPT Auto
'81 TR7 DHC