Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Dual circuit brakes

Is the dual circuit brake master cylinder fitted to later cars a staight swop for the single cicuit one on a 69/70 B (UK Spec)roadster. Or if not, what mods are needed. (No servo is /will be fitted) Thanks Michael
Michael Beswick

The American non-servo dual master is definitely an easier prospect than the later servo assisted dual master fitted to later American and UK cars, that involves major pedal and pedal box changes. Having said that I've not seen or heard of such a conversion so there may be issues I'm not aware of.

But I don't think the non-servo dual master was ever used in the UK, have you managed to lay your hands on one? Different pipe-work of course, and some if not all of the non-servo masters used a remote pressure imbalance switch and manifold.

Paul Hunt

As I said to Michael on the MGOC BBS, I have got the late in-line servo/ dual circuit brake master in my 1969 car. It isn't straightforward as the square hole that the pedals drop down through has to be enlarged and there needs to be a slight depression hammered in the inner wing to make room for the servo body. You have to use the pedal box and pedals that come with the later car as well.

I wasn't aware that there was another dual circuit cylinder design fitted to US cars. It isn't shown on the Moss web site.
Mike Howlett

Mike, the non-boosted dual circuit system was used in US cars 1968 through 1974.
John H

Aha - the plot thickens! I was hoping to use the dual m/c shown on Moss UK site but without the servo. However on the Moss US site there is a dual m/c desiged/supplied for use without the servo. So I suppose the real question is whether this US dual m/c is (a)interchangeable with a single m/c and (b) what if any issues around it being fitted to a UK rhd car as opposed to a US lhd. Oh and $160 +freight!

Paul- I haven't found one yet, but I haven't looked very hard!

The purpose is simply safety. I work as a MOT tester. (Annual "safety" check in UK for cars over 3 years old.) The idea of a seal going on a single line system and having no brakes except the hand (parking) brake seems a bit unpleasant! An alternative would be to replace the current (old) single m/c despite there being no signs of leaks / discoloured fluid. Then of course the slaves and caliper pistons..... Or should I just live dangerously?!!!!
Michael Beswick

Michael, In all of my years of working on cars and espessialy braking systems, I have seen almost all dual circuit master cylinders fail completly when only one circuit is breached. The dual master cylinder, in my opinion is more of a false hope rather than a true safety feature. In racing, two seperate master cylinders are required to prevent a catastropic failure. RAY

Michael the US dual master cylinder without the attached servo, is a straight swap for the original UK spec single circuit master cylinder.
I actually need a seal kit for one of these if anyone in the UK can help, either with a part number or even a kit. Paul??


Colin Parkinson


I have the utmost respect for your automotive opinions, but I will have to disagree with you on this one.

I do believe that the two separate master cylinders is probably better than a unitized dual master, but I also believe the dual master is a vast improvement in safety over the single system, and I believe it is a very worthwhile upgrade.

I have never lost all of the brakes on a car with a dual master. I have at times lost half of them. On the other hand, I have lost brakes on single systems probably five or more times.

Also, I have some young motor head friends who look at me with puzzlement when I talk about complete brake failure because they have never experienced it. I don't think it is their youth that is responsible; I think it is the youth of the cars they grew up with.

I do hope to make another west coast trip this year, so maybe we can grab a beer at the North Coast Brewery. My fuel pump failure on the BGT just south of Leggett prevented that meeting in January 08 when I was out there last.


C R Huff


Angloparts ( do have the seal kit you are looking for.

Picture of the dual master:


Per Sj÷berg

I have to throw my hat in with Ray.

Whenever I experienced a brake MC failure
on my '71 B/GT - it was always a total loss of
brakes. I've never experienced a partial failure.

Real fun! Always keep your parking brake cables
and linkages in top order!

Thankfully, it doesn't happen often.
Daniel Wong

midgetdea of a seal going on a single line system and having no brakes except the hand (parking) brake seems a bit unpleasant! "

Bear in mind that if the front circuit fails you only have the same retardation as is available from the handbrake anyway! This could well explain the comments by Daniel and Ray, as suddenly finding you only having rear brake retardation when using the foot brake pedal is certainly going to cause one of those "oh sh*t!" moments. If the rear circuit fails you quite possibly would only notice it from the slightly longer pedal. For this reason if you are going to the bother of retro-fitting a dual servo I would also consider plumbing it for diagonal circuits and not front and rear.

The dual master non-servo seal kit is 18G 8587, listed by Sussex Classic Car parts (
Paul Hunt

Thanks to you all! I will investigate At Sussex car parts and see if the Midget is the same. If not, Moss US looks possible.

Is the brake front to back "bias" achieved thru the relative size /effectiveness of the front calipers and rear slave cylinders? Or is there something designed into the dual m/c? I think Paul's idea of diagonal is better than front / rear. thanks again Michael

Michael Beswick

main differece would appear to be the angled reservoir faces the "bellows and pin" end for the US B and away from the bellows and pin on the UK and US midget so maybe clearance issues? Michael
Michael Beswick

Charley, I'm glad that we can agree to disagree. I've owned my '67 Roadster since '72 and have never experienced any brake failures. This may have something to do with the fact that no one works on any of my vechiles but me. I tend to err on the side of caution when it comes to maintenance. I have a history of everything done to my car dating back to '83 on my computer, before that written out in longhand. Speaking of Leggett, I had the MG out last week and cruised through there. Hope to see you the next time you come out. RAY

I had a rear flex hose fail on a trip one time in my 72 MGB, and was thankful that it had the dual circuit master cylinder, as it provided me with "some" braking ability. Surprising how much less braking there is with only front brakes, and I can see how it could be that much worse with only rear brakes. But better than none at all. The pedal felt like it was going to the floor when it first happened, then became firm again once the rear piston contacted the front piston and applied the front calipers. It was an "oh-sh*t" moment!
I agree with Paul in that losing the front brakes would result in the maximum braking ability being the same as the handbrake, however I feel the pedal is much better to modulate while in an emergency than the handbrake ie if the tail starts coming around etc.
Two single master cylinders side by side would be great as Charley says, and plumbing diagonally with a dual circuit system would probably improve performance if on half failed, but I would think that would be dicey on a wet or otherwise slippery road.
I think if it were me, I would switch from single to dual piston master cylinder for the added safety margin.

Erick Vesterback

Yes, unlike the original Mini with drum both ends and a proportioning valve the MGB achieves the correct front to rear balance from the relative cross-sectional areas of the calipers and slave cylinders.

FWIW GTs have slightly larger slave cylinders giving slightly more rear braking effort due to the greater weight of the car. But paradoxically V8 GTs use roadster cylinders, as their stiffer suspension makes locking over undulations more likely. The different slaves have locating pins in slightly different places, hence different backplates to accommodate them.
Paul Hunt

This thread was discussed between 26/02/2009 and 01/03/2009

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.