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MG MGB Technical - Brake sensor

There is a plastic sensor under the master cylinder on my 76 MBG that seems to be leaking brake fluid all over my wheel well. My local parts store has no listing for this. Can anyone help with a source and part #. Also can this be repaceled without bleeding the master or not? Its a low pressure switch I bleive so does this have any function other than just turning on the idiot light and can it just be blocked off if the switch is no longer availble? TIA
Dave
Dave Noe

It is the brake imbalance switch, all it does is operate the warning light. I doubt a local parts store would have a listing for *any* MGB parts after all this time other than consumables like filters and brake pads/shoes, same goes for Rover's own parts suppliers. Specialists like Moss have it e.g. http://mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29030#40 item 40.
Paul Hunt

Thanks Paul for the link thats it exactly. Does anyone know if this is just replaced or do you neeed to bench bleed the master after replacement?
Dave Noe

Dave,

The plunger on the bottom of the switch bears on a waisted shuttle in a balance tube between the 2 circuits. This shuttle has an O ring seal around each end. If brake fluid is making its way out through the switch one or both of the O rings has failed. These need to be changed first. If the fluid has damaged the switch then the switch need to be renewed as well.

The alternative is to put a bolt in the place of the switch. However, you then loose the warning facility.

David
David Witham

Dave,

I have an advertisement for my TR6 electrical repair manual online at: http://members.aol.com/danmas6/

As it happens, I have a page from the chapter on the brake warning switch in the ad which you might find helpful. Even though it's for a TR6, the PDWA is the same on an MGB
Dan Masters

"However, you then loose the warning facility."

Yes, but by the time the pressure has dropped sufficiently to let the shuttle slide over, one would have hoped you'd have noticed something wrong with the brakes by feel. The indicator can't *warn* you, because by the time it lights up, the problem has already manifested.

I'd say you could get away without the switch quite well. Except that seeing the bolt in the switch hole might make you feel like a Dpo.
Matt Kulka

The shuttle will move as soon as there is almost *any* fluid loss from one circuit. In the majority of cases this will be before you lose braking on that circuit, that is why it was fitted. Only if you get a catastrophic failure like a pipe breaking would I expect you to encounter brake imbalance at the same time as the warning.

Paul Hunt

Once again, corrected by Paul Hunt. And rightly so, apparently.

Paul, are you related to my wife?

Matt Kulka

This thread was discussed between 21/01/2004 and 26/01/2004

MG MGB Technical index

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