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MG MGB Technical - Brake master cylinder

My car is a February 1974 US spec roadster. As a result it has a twin circuit non servo master cylinder.

I have owned the car for 13 years and in the early days of ownership I rebuilt the master cylinder with a new seal kit. While the car has only done 15000 miles since then I suspect the cylinder may need doing again. (Soft pedal, no air in system and no visable leeks)

I am considering replacing the master cylinder with a new one rather than changing the seals. Would a single circuit master cylinder be a straight swap, other than the pipe work? Or were there changes to the pedal box or pedals when thi dual circuit master cylinder was fitted.

(For the benefit of other people in the UK the dual circuit master as fitted to my car is totally different from the one with direct acting servo used on late rubber bumper cars.)


David Witham

Why do the swap? A dual circuit is far better than a single circuit, especially with brakes.

There's a reason the US DOT required dual masters. Ever loose your braking system or have a hose fail?
Mike MaGee

Mike you say "Why do the swap?"

1. May be time for a new master cylinder rather than another seal kit and these dual circuit ones are very difficult to get over here, as never used on the UK cars, and if I can get one it will be expensive.

2. At some point, perhaps this winter, I will convert the car to RHD. If I continue with the current dual circuit master it will be almost impossible to fit the pipes to it when it is insitu.
David Witham

David, Moss shows different part numbers for the push rod, brake pedal and pedal box cover. I think the cover difference is a fitting for the stop light switch. The push rod is probably needed to fit the replacement cylinder. I have no idea what is different on the pedal. Bob Thompson has made the same change you are planning. He will know what is necessary.
I agree the MGB dual master cylinder is expensive, they are $239.95 at Moss USA. It's also more difficult to rebuid than most dual cylinders. Maybe Bob will respond.


Clifton Gordon

Since the split braking system used on the MGB uses one half to feed the fronts and the other half the rears it is only protecting you against a rear circuit failure. How often does that happen, and the UK MOT should pick up dodgy hose and pipes long before failure. Since UK spec carried on with single circuit for many years personally I would have no qualms about converting back to single-circuit. US spec also got the mechanical brake light switch (mounted on the cover) some time before the UK. A pre 77 rubber bumper pedal box cover (or V8) would have the fitting for the switch but not the servo, which would be preferable to retro-fitting the hydraulically actuated switch, but you would almost certainly need the matching master cylinder frame. The clutch master cylinder should change over.
Paul Hunt

I, too, have a dual-line master cylinder that needs rebuilding or replacing -- 1969 MGB that I've had since 1979. Master cylinder never rebuilt, but car has been sitting about 6 years this time. (It was idle for 5 years other times). I would rather try rebuilding than spend the $200-plus on a master cylinder right now. Having trouble, though, understanding how to remove box with both brake and clutch cylinders. I'll have to do that whether I rebuild or replace. Is removing the box that bad? Is my fear overrated?
Mike Kaylor

Mike, once you have disconnected the pipes it is straight forwards. If you have the factory manual it cover it reasonably well.

The pedal box cover has 4 screws. The brake lines are undone from the side of the master cylinder and you may find it easier to undo the clutch master from behind the dash, through the hole with the big grommet in it. Then there are 4 bolts to undo and return springs in the footwell. After that the pedal box lifts out from the engine bay.

Going back to my original question we don't have a complete answer yet.

Clifton, I believe the single master would come with a push rod. I know the pedal has a different part number but I do not know if the differences in the pedals is significant. Secondly, I don't know if the pedal box frame is the same.

You mention Bob Thompson having done this. Do you have an email for him.
DJ Witham

David,I sent you an email with Bob's email address.

Clifton Gordon

Mike, Some owners have removed the master cylinder without removing the pedal box assembly. I believe the difficult part is getting the mounting bolts back in. Several different methods have been used but I don't recall the specifics. The information is in the archives. I removed the pedal box when I changed mime because the mess under the pedal box needed cleaning and painting.

If your master cylinder has been idle for six years it probably will be pitted. I rebuilt mine and six months later it was leaking again. The MGB dual cylinder has two bore diameters and is very difficult to hone and repair correctly.

Clifton Gordon

Well I followed Clifton's advise and I contacted Bob Thompson. He confirmed that the cylinders could be swapped over in the same pedal box frame and that I could continue to use the existing pedal.

His way of resolving the plumbing was good as well being simple. You take the shuttle and switch that are used to detect imbalance in brake pressure out of the brass fitting they are in. Cap off the switch hole with a suitable bolt and in the same way cap off one of holes for the line in from the master cylinder. I used copper sealing washers on these bolts.

Unbolting the pedal box frame gave enough movement to allow the old master cylinder to come out and the new one to go in without disconnecting the clutch hydraulics.

As I had some spare unused brake pipe I made up a new pipe to run from the master cylinder to the junction block.

I am sorry I can't tell you if it works yet. Although I am near to completion, my wife changed the time table for painting our daughter's bedroom!!!

David Witham

OK, an update:

I managed to remove master brake cylinder with pedal box and clutch cylinder. Loosened the brake lines from pressure valve because I couldn't reach connections on side of master cylinder (after about 4 frustrating hours). Tried to rebuild the master cylinder and managed to get Spirolux ring, spring, first rubber washer and first circlip. Never was able to budge the nylon guide bearing. Gave up and put the car back together. Still no pressure in the brake lines.

Is it just time for a new master cylinder?

Mike Kaylor

Mike, Yes, you need a new or rebuilt master cylinder or have someone rebuild yours. I agree the MGB dual master cylinder is difficult compared to others I have rebuilt.

Clifton Gordon

Just an added note on the duel line system. I have found them to be extra bad about leaking if not used much. I got warranty on a new cylinder from Moss do to leaking but have since deceided that the problem is you must work these cylinders to keep them from leaking. Bob Thompson
RHT Thompson

This thread was discussed between 08/07/2005 and 26/07/2005

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