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MG MGB Technical - Brake proportioning valve

Fixing a brake system on a 1979 Spitfire that has been in a garage for 9 years. Proportioning valve appears to be the same as the later MGB's.

Shuttle was froze, so pulled the bolt out of the end and added a little penetrating oil. Let is sit in it for a while and it freed up. Cleaned the system out with brake cleaning fluid.

My question is how free should the shuttle be?

Should it slide easily or should it take a bit of push to move it? I know it has O rings inside, but not sure if it is moving smooth enough.
Bruce TD4139 Cunha

It's not a proportioning valve, it's a brake fail switch.
The piston only needs to be jammed in the middle for the brakes to work just perfectly. Make sure the O rings are present too, or you'll leak everywhere and have no brakes.
I guess it needs to be present to pass whatever inspections you have there? If not, just bypass it.

I don't think you can bypass the fluid/mechanical part of the switch, but you could disconnect the wiring or remove the bulb. Depends if the electrical functioning of it is inspected as Roadwarrior says.

The earlier remote manifold and shuttle moves under unbalanced brake pressure, sticks there, and has to be moved back by unbalancing the pressure in the other direction. The later shuttle that is part of the master cylinder latches over and the switch has to be unscrewed for it to recentralise, therefore it should move freely, presumably with pressure from a fairly light spring on either side.
Paul Hunt

Be sure to replace the o-rings while it is out and apart. They will leak otherwise and you will lose brake fluid through the port where the warning switch goes. Unless the switch port has been completely sealed with a bolt and copper washer, it will be important to replace the o-rings, especially after exposing them to penetrating oil.

The o-rings are an ACE hardware item and cost pennies.

I have the same brake warning failure on my midget, on the B and it was on my 1985 Range Rover as well.
Glenn Mallory

Thanks all. Good information and a better understanding of what this is and how it works. So the purpose of the valve is simply to turn the brake warning light on if one side fails? I assume if there is a leak on one side, the piston is pushed over towards that side and turns on the brake system light. WIth the piston in the middle, both sides have the same pressure and the piston stays there.

Is that about it?
Bruce TD4139 Cunha

Yes, that's it.
Dave O'Neill 2

This thread was discussed between 24/09/2015 and 26/09/2015

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