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MG MGB Technical - 80 MG B brake mc shuttle valve o-rings/leak

Hi everyone!

I could not find any conclusive answer to my problem in older posts. Appreciate inputs.

Just finished a mc overhaul (repair kit). Got now near to perfect braking action (1980 MG B rectangular mc reservoir).

Brake warning switch now leaks and/or warning function lost. Repair kit contains two small and two bigger o-rings.

When taking the 'pressure differential piston assembly' out of its bore for the o-ring replacement, there were 2 o-rings installed. The assembly came out of the casing relatively easy and the switch would not leak.

1st repair try:
After replacing the forward o-ring (in its grove) with one of the new o-rings and placing the second o-ring at the same position as we found the second o-ring after the pull out (on the shaft with no groove), inserting the assembly was hard and the switch would leak.
After pulling out again, we found the new rear o-ring sheered off and damaged.

2nd try:
Installation of second o-ring at the very end of the shaft in the groove. Assembly could not be inserted fully.

3rd try:
At the 'original, as found' second o-ring position we placed both old and worn o-rinds and at the very end (in the groove) a small new o-ring.
The assembly did move in, but not completely (pressure on the spring). The assembly seems to be stuck in a slightly wrong position (the 'bridge' for the pressure switch is not visible through the switch hole). Although the switch does not leak any more, the safety function seems to be lost.

Where has the second (rear) o-ring to be placed for installation and which o-ring size has to be used?

1. At the very end in the groove?
2. On the shaft (perhaps with a small new o-ring, stretched to get it thinner)

Sorry for the long story, spend many hours on it already.

Sketch attached.

Thank you!

Reto Schlumpf

Haynes has a drawing of this valve (attached), however it doesn't show the extension on the right between 'Found old O-ring' and 'Why is there a groove here' that you seem to have. It also appears to show the two O-rings being the same size, and fitted either side of and equidistant from the slotted section that the switch sits on.

Incidentally I'd have said that the switch should sit over the narrow central raised party, so it can drop into the grooves either side depending on which circuit fails and hence which way the shuttle moves. If it's not moving that far now then it's probably completely jammed. Does the shuttle go all the way in with no O-rings? Have you checked there is no broken O-ring stuck in there? It is possible to put some kind of gauge in there to determine the size of the hol in various places, and compare that with the size of your shuttle with O-rings? Did you lubricate the O-rings and hole with brake fluid before fitting?

PaulH Solihull

Paul. The Haynes's diagram is incorrect. Reto's drawing conforms with internal design of the factory master cylinder I sectioned so people could see what the set up looked like and how it worked.

Reto. The proper set up is with a single ring on either side of the two grooves that the pressure failure switch would drop into. There is something within the cylinder's bore that is causing the rod assembly to be too far forwards--at least according to your drawing. The two O rings fit into two small grooves on either side of the two much larger grooves. The center, between the two larger grooves, is where the plunger of the pressure failure switch makes contact. You show the entire rod assembly shifted too far forwards with the pressure switch plunger making contact far rearwards (because the rod is too far forwards) of where it should.

Without seeing the master cylinder and being able to take it apart and inspect it in person, it is hard to offer definitive advise.

The O rings go on either side of the large grooves and fit into small grooves. The O rings should be just large enough to fit into the cylinder bore with some resistance--your should have to apply a little brake fluid to the rings for lubricant to get them pushed in.

How did you clean the master cylinder when you did the rebuild? The areas forwards and rearwards of the pressure switch are fluid bearing (containing) areas and may well have some rust build up. It does not take a lot of rust, or rust damage, to prevent the rod assembly from moving fully backwards and seating properly.

On your lower drawing, you show that you have placed an O ring on the small V groove at the rear of the rod assembly. From looking at my cut-a-way example, I see that the groove is directly in line with the hole in the main reservoir and the upper brake line fitting. The hole between the master cylinder pressure switch assembly and the fitting on the hard brake line is only about 3mm in diameter. It looks like the purpose of the groove, which you have sealed with an O ring, is to allow fluid to flow from the main bore, through the pressure switch bore, and into the fittings for the hard lines. By fitting the rearwards most O ring, you have restricted the flow of brake fluid to your front brake system. I would suggest you remove it.

I detail strip my master cylinders and bead blast them before deciding whether to rebuild them or not. After bead blasting and washing and drying, I lightly flap sand the interior of the bores with 320 grit emery cloth. (A flap sander is a length of steel rod with a slot cut into one end to insert a length of emery cloth. The other end is inserted into a drill or die grinder and, as the machine is rotating the flap sander, the emery cloth and rod are moved backwards and forwards along the center line of the work piece. Lightly is the operative word with the light polish showing up any excessive pitting or other damage. It, also, cleans up any very light damage which should not cause problems with the rubber parts having a good seal.)

Please let us know what you find. About ten years ago, I replaced all of the master cylinders on our Bs with new ones. My thought was that the old cylinders, under unknown conditions of maintenance, had lasted for 25+ years. New components, with a regular maintenance program, should last the rest of my life. An added benefit is that, with new components, if they were ever to be rebuilt, we would know the history and be able to know that we were not trying to rebuild a system which had been rebuilt several times before.

Good luck. Les

Les Bengtson


Thank you for your comment and drawing.
Unfortunately I have the same drawing in my workshop manual. It is not reflecting the design of my mc with non-wedge type reservoir.

We had the mc in and out 3 times now with bleeding, that's why I was hoping someone would have a cross-section drawing of my assembly and we could fix it without taking it out again.

You are right might have to take it out again for proper investigation.

Reto Schlumpf


Thank you for your comment.

I fully agree with all your findings and observations, except there is no groove for the rear o-ring. When we de-assembled the valve, we found the second, rear o-ring just sitting on the long cylinder (no groove).

That is why I am confused where to put the rear o-ring.
Technically I have never seen an arrangement like this (no groove for o-ring).

I read in another forum that the purpose of the V-shaped groove (at the end) is different.. (You mention that it could be for the purpose of b-fluid cross flow and should not have an o-ring. I think so too!). In that forum they suggested to fit an o-ring there so it would slip forward on the cylindrical part during/after installation.. But it seems to me, by going that way, the "end" o-ring does NOW not allow the assembly to move to the end position (That very o-ring did not slip forward)..

Would it be advisable to replace the presently installed (2) old o-rings (on the cylindrical part, which seem to hold the b-fluid back) with (1) NEW small o-ring (very stretched, so it would fit in the bore and remove the end o-ring in the V-groove)?
Or is the risk of braking that stretched o-ring at a later stage to high?

- The bore has no rust
- We cleaned the mc with brake cleaner
- For assembly we used brake fluid

Perhaps I should have bought a new mc instead. I really would like to have the valve working properly and not just replace the switch with a plug (to seal it off).

Reto Schlumpf

This thread was discussed between 07/12/2012 and 08/12/2012

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