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MG MGB Technical - hand break light

hello can any one tell me if the hand break light is a warning for any thing else as its on all the time thanks daren
daz

There is a switch on the side of the master cylinder, that comes on if there is a problem with one of the brake circuits. It makes the handbrake light stay on.
Sometimes, adjusting the rear shoes will fix it.

If you disconnect the switch on the master cylinder, and the light goes out, then that is the problem, and you will need to get the brakes sorted out.

On the other hand, if you fit a new carpet, the handbrake wont go right down to the little switch, and the light is on all the time, like in my car before I cut a piece of carpet away to fix it. The drivers seat has to come out to find the handbrake switch.
Martin Layton

Has the pedal pressure or braking distance changed recently? Ray
RAY

passenger seat?


Dan Robinson

BRAKE BRAKE BRAKE!

The Haynes diagram for 'later UK' cars only shows the light being lit from the handbrake switch, and if the handbrake is down when you crank that will light it as well. However UK cars got the split braking system in May 77, so one would expect the imbalance warning circuit to have been fitted as well, which also lights the lamp on American cars.
Paul Hunt 2

To put it simply Daz

1) Is there a switch on the side of the brake master cylinder? If so unplug it.
Does this put the light off when you are driving?

If yes, brake imbalance. Adjust rear brakes, reconnect switch, try again, if light still on, something is wrong with the brakes, and you need to do more checking.

2) If no, make sure that the handbrake lever is pushing the switch all the way down. You need to remove whichever seat is next to the handbrake, and look for the tiny switch at the bottom of the pivot. With the ignition on, pushing this switch should put the light out. If not, check the switch.

As Paul says, the light is supposed to come on when the starter is turning, there is a diode shown from the starter terminal to the light. (If the diode fails, either the lamp never shines=open circuit, or the starter will run all the time the handbrake is up = short circuit)

I would only use the Haynes diagrams as a guide, rather than a Gospel, you need to check what is actually fitted to a particular car.
Martin Layton

I'm not sure simply adjusting the rear brakes will recentre the shuttle switch. Indeed if it has become uncentred it indicates there is a problem on one or other circuits which must be investigated first, that is what it is there for! Whilst in theory more wear on the rear circuit could cause a fluid imbalance, as it is not self-adjusting like the fronts, I've never heard of this as a cause of the light coming on.

Bleeding first one circuit then the other will move the shuttle from one side to the other, passing through the 'off' position on the way. Whilst bleeding someone needs to be watching the warning light, ignition on, coil disconnected or engine running, shout as soon as it goes off, then you immediately close the bleed nipple. If you don't close it fast enough it will move too far and come back on again, and you will have to try again bleeding the other circuit. Of course this should only be done after adjusting the brakes, as adjusting them after may move the shuttle off-centre again and you will have to bleed again.
Paul Hunt 2

I was thinking that if the rear brakes were well out of adjustment, the master cylinder rear piston could bottom out, so that the front circuit could have more pressure than the rear, and trip the switch.

If the switch has a memory, (the piston moves over and stays put), then even pressing the brake with a drum off, or a pad out could have tripped the switch, so it will just need resetting, by careful bleeding, following the manual.

In any case, Daz needs to find out why the light is on, whether due to the hand brake switch, the pressure differential switch (if he has one) or a wiring fault before he can proceede further.

Paul,

I don't look forward to bleeding the brakes if I have to shout "now!" at the right point, and be quick with the spanner :-)
Martin Layton

thanks for that im on the case daren
daren

Martin - especially if you have to run from the dash to the caliper! :o) I think the rear brake adjustment *can* be a valid cause, although I've never heard anyone report it. Because the rears aren't self-adjusting like the front and cause a long pedal (on a single circuit system) I'm assuming the front circuit pressurises whilst the rear is still filling the slaves, and that pressure imbalance is what moves the shuttle switch. I'd expect the pedal to bottom before the master piston did, like the single-cicuit master does, for safety reasons.
Paul Hunt 2

This thread was discussed between 03/11/2006 and 06/11/2006

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