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MG Midget and Sprite Technical - Brake issue

The brake master cylinder went out on me and after replacing it with a new one I now have front brakes, but no rear brakes. There is a switch under the master cylinder and I am wondering if this is bad or if there is any way to reset it.

BTW, 79 midget. Thanks for any suggestions or tips as to where to maybe find a new one since moss doesn't show it as available.

G Laird

Not to worry, this isn't a big deal. That's the brake compensator and it's designed to move a metal shuttle located inside the fitting in the event part of the dual braking system fails. You still have some limited braking capacity in an emergency situation. The shuttle will ground out the switch that screws into the fitting illuminating the warning light on the dash in the event of a failure of part of the dual brake system.

You need to get the shuttle to move back to where it was and unfortunately I can't recall for any certainty how to do that. You can try bleeding the system diagonally starting at the right rear then left front and see what happens. Once you get it set up and the light is off, bleed from the longest line brake to the shortest. If it illuminates, just bleed the opposite end until the light goes off and stop at that point as the shuttle is now reset.

I'd consult my factory repair manual but it's at my other location but I'm sure someone will be able to help you along.

Anyway, that switch can be rebuilt if need be and Moss were selling that for in excess of $400.00. Sometimes the shuttle will stay in place due to it not moving but it's not difficult to reset and you don't have a huge problem here to correct.

Clive Reddin

one way to return the shuttle to its center position is to remove the electrical switch from the PDWA (pressure differential warning assy), and through that hole shift the little shuttle with a small screw driver.

No fluid will leak out when you do this, as the shuttle is sealed by o-rings to prevent any fluid contacting the switch.

If you have only front braking now, then the shuttle has been shifted forward in the housing by pressure imbalance in the system, and needs to be moved back to the middle (the front brakes are plumbed through the rear three ports of the housing, and the rear brakes through the front two ports).

This issue is not uncommon when bleeding the brakes using too much force. It is handy to work with minimal pressure throughout the bleeding process to avoid moving the shuttle accidentally.


Norm Kerr

>>> No fluid will leak out when you do this, as the shuttle is sealed by o-rings to prevent any fluid contacting the switch. <<<

Unless the O-rings have gone south. Ask me how I know...

Interestingly, the bleeding procedure as outlined in the factory manual entails attaching bleed hoses to the front and rear brake on each side of the car, and bleeding simultaneously. That way the pressure stays equal on either side of the shuttle, and it doesn't move, maybe.

And yes, the entire switch assembly is apparently made of gold, and is ridiculously expensive. But a set of new O-rings (i.e., PDWA Rebuild Kit) costs pocket change. In fact, I'll bet if you remove the little shuttle bit and take it to your local hardware emporium, you'll be able to find a couple little bitty O-rings right off the shelf.

BTW, I bought the rebuild kit a few years ago, and it's still in a bag in the boot. I can't bring myself to open the brake system for something like that. I plugged the switch hole with a bolt and copper washer to hold the fluid in, and it works fine...

Gryf Ketcherside

Haynes documents a bleeding sequence that I have used to successfully re-center the shuttle. I don't have my Haynes here at work, so I won't try to remember it for you.
David "brokem rememory" Lieb
David Lieb

Thanks for all the suggestions. I did what it says in the manual and still no luck. Am frustrated now. Will probably have another go at it tomorrow.
G Laird

Was messing with it more tonight. The shuttle is indeed moved forward in the housing. Something is keeping it pushed forward. I am able to move it back with a screwdriver butsomething is forcing it back forward. I took off the rear brake line from the brake master cylinder thinking there was a build up in pressure and that is not the case.

Still stumped.
G Laird

The switch itself isn't difficult to clean. Get the rebuild kit and take it apart. The shuttle comes out once the large hex is removed. (You can see it on the assembly). Sorry I don't have the manuals in front of me or my spare switch assembly, but there's some O rings and probably not much else in the kit. Don't forget to remove the brake warning switch though or the shuttle won't be able to come out.

I wonder if there's possibly some pressure leading to the rear brakes instead of from the master cylinder?
Clive Reddin

This thread was discussed between 22/03/2011 and 11/04/2011

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