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MG MGB Technical - Brake warning light - '69
|Need a little advice. The other day in the garage, I noticed the red brake warning light on (when the car was off!). I was going to take a look for a problem, but it went out by itself.|
Took the car out for an afternoon's drive yesterday after checking out the brakes and a couple of times the warning light came on for a while and went out by itself. There was NO problem with brake operation. That happened in the first hour of the drive and the rest of the day there was no problem.
According to the manual, that warning light should only come on "with brake application" when there is a problem detected. Well, I can't find any brake problems and that light was on steady while driving, without brakes applied. The light was slightly dim and when I pressed the test button to turn it on, it would brighten up some.
I'm thinking there's a short somewhere that's providing voltage intermittently. Anybody got any insight?
('69 doesn't have power brakes)
|Doesn't have power brakes, but the '69 US spec did have dual circuits didn't it? In the dual circuit master cylinder there is a shuttle that is pushed one way or the other if one circuit is weak, and this lights the lamp. It might be stuck I guess, but why it's on when the key is out, I can't tell you.|
|Mike, on the type that Roger has the shuttle and switch are in a seperate fitting.|
Roger, I had a similar problem on my ex US feb '74 car. The shuttle Mike mentioned is in a brass fitting on the drivers side inner wing. There is a switch in the fitting that waist of the hour glass shaped shuttle. The shuttle has an O ring at each end and is in a balance passage between the front and rear circuits. A pressure difference moves the shuttle and operates the switch.
On mine brake fluid was coming up through the inside of the switch because one of the O rings on the shuttle saw letting a small amount of fluid past. So whilest the switch was being triggered the fluid inside it was causing significant resistance and hence only a faint glow at the warning light.
Solution is a new O ring seal and you may have to replace the switch as well.
|David, thanks for the advice. I'll order the parts - might as well replace the whole unit unless it's prohibitively expensive. I think it's odd though, that it came on by itself in the garage, when the car had not been running. Since the shuttle is or should be, relatively level, I wouldn't think it would move while sitting in the garage!|
Thanks and I'll take your advice and run with it!
|The brake test/indicator light on the dash is powered by a purple wire so if the shuttle has moved to turn on the light it will remain on unless the fuse for the purple circuit is removed or the battery disconnected.|
A short in the black/white wiring, unbalanced braking between the front/rear and perhaps a leaking seal in the shuttle could turn the light on. I would check the position of the notch in the shuttle first to see if it has moved to turn on the light. Remove the plastic switch on the valve body if a seal is leaking there will be brake fluid under the switch, if it's dry the seals are probably ok. You should be able to tell if the shuttle has shifted with a small nail or punch inserted in the hole for the switch plunger. You should be able to shift it to the center with the nail. The alternative method is to have someone push the brake pedal down and watch the light while a brake bleeder is quickly opened and closed. If you have guessed correctly on which end has the most pressure the light should go out if you are quick enough on the bleeder screw.
If you have enough leakage in the system to have an unbalanced condition the light will come on again. Your brakes may feel ok until one side of the brake reservoir runs dry. The shuttle does not shut off the defective end of the braking system, it tuns on the light to warn of a posible problem.
For a leaking valve the plastic switch can be replaced with a 3/8" NF short bolt and a copper washer. Your brakes will work ok but the test circuit will not.
Here is s shuttle from a Pantera site, it's almost identical to the one used on the MGB. Click on the drawing to see the normal and unbalanced shuttle positions.
|Clifton, thanks very much. That's great information and I'll check it out right away. I just visited the Moss on-line catalog and HOLY CRAP! It looks like the switch and body assembly, part 141-710, is $543... yep, that's what I said! Five hundred and forty three US dollars! That's not gonna happen in my lifetime, I don't think!|
|Roger, you most likey don't need the body and maybe not the switch either. New seals can be purchased and installed on the slide valve. There were at least two types of valves. One takes the correct size O ring mentioned earlier. The other takes a cupped lip seal like a brake cylinder the center cut out. I think the switch is now made of unobtainium. Later models are a little diffrent.|
|Roger, If you don't have fluid under the plastic switch the seals in the valve are probably ok. If it does leak the kits aren't too hard to replace, I did mine a few weeks ago. Your car may have the early style shuttle, it takes a different kit than later cars. Yes, I'm aware of the cost of the new valve assembly. Used valves show up on eBay frequently. |
|I've got the rebuild kit coming just in case and if I don't need it, I'll keep it for when I might.|
Clifton, I'll look for any sign of fluid that may have passed the seals. I also presume that if there is no fluid leakage, there's a reason the shuttle moved, since it's there to detect a pressure differential. If that's the case, I'll be real curious to know what it might be. All lines are new, calipers rebuilt, etc, including the hard line through the body, as this is a Heritage reshell. I hesitate to put a screw in the valve and pin the shuttle in the center if I'm just covering up a problem, know what I mean? Am I off base?
Thanks a lot, all you guys. The knowledge available out there is an unbelievable help.
|Hi all . The AQustralian 69 roadster has no brake warnig light at all..Single cylinder as well..I always knew we were behind the times.....|
|Cactus, the UK cars also didn't get dual circuits for years after the yank cars had them.|
|Yeah, but you guys got to keep those nice metal dashes and we got these really ugly plastic things!|
|I suppose we can;t have everything..Looking at the costs of the brake parts above, i am glad we are behind the times.Also glad we kept the metal dash.|
|Just thought I'd post my final results following all this great advice so you guys will have it for reference!|
I pulled out the brake warning light valve that's mounted to the left fender, in the engine bay. There was a small amount of brake fluid under the switch, indicating that the seals on the shuttle were leaking a bit. I tested the switch while out of the valve, cleaned up the valve, replaced the O-rings on the shuttle and put it all back together. Hint: before disconnecting the brake lines from the valve, get yourself a plastic garbage bag and slide it under the valve, between the valve and anything else, so you don't get brake fluid everywhere... which takes paint off.
I made myself a tool - basically a 5/16" bolt with about the first half inch of it ground down so that it would fit through the small hole under the switch and pin the shuttle in the center and not allow it to move during bleeding. I threaded this into the valve, in place of the electrical switch, and then bled the brakes. Worked like a champ.
After bleeding the brakes, I pulled out that bolt and put the switch back in and it all seems to be working fine now. The brake warning light on the dash works - at least it comes on when I push the "press to test" button.
Thanks to all who provided advice!
This thread was discussed between 20/06/2005 and 27/06/2005
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