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MG MGB Technical - Twin Circuit Master Cylinder

I'm confused about what bits go with which. I am rebuilding a 1969 GT, which being a UK car has single circuit brakes with no servo.

It might seem a good idea to fit twin circuit brakes, but which master cylinder would fit with or without a remote servo? I'm not a great fan of the servo and reckon the brakes work fine without one. Can anyone give me the part number for the twin circuit master cylinder please? I don't want to use the one with the direct acting servo set-up.

Would I have to change the pedal box to change to a different master cylinder?

Mike
Mike Howlett

Mike. The US spec twin circuit master cylinder was used from 68 to the mid 70s. Not sure of the exact year of the changeover. By 77, they had a twin circuit with a servo/power brake booster mounted between the pedal and the MC. Do not know if this set up would work with your RHD car. The 68-7? system should work and, if necessary, you may be able to find the necessary parts used. (I am thinking of the four way connector as being used. The cost of a new one and switch is about the same as a new US spec dual master cylinder.)

This having been said, I have never seen a brake problem that was solved by the dual circuit master cylinder. When one of the braking circuits goes out, say the loss of a rear wheel cylinder, the remaining braking force is less than marginal. It does give you a very limited amount of braking power if you drive very slowly and carefully. So does engine braking and the parking brake. I mention this based on actual experience with a blown rear wheel cylinder. I am not sure I would trade from the simplicity of the single circuit master cylinder (which some feel is more reliable) to the increased complexity of the dual circuit MC unless I was required to. Les
Les Bengtson

Les,
Your comments are very interesting. I've often wondered how much residual braking would be left if one circuit failed. Certainly I've been using my '71 roadster daily for 10 years with the single circuit brakes never giving a moment's concern. Perhaps I won't bother....

Mike
Mike Howlett

Mike, I had an interesting braking experience with my single servo '71 B back in 1985. Lets just say I had to change my trousers that day and I was lucky to have that center area runoff.

Luigi
Luigi

The changeover was approximately the same time as the rubber bumper.
Use Moss # 180-765. It is bolt-in with the exception of having to run new plumbing. The light switch is mechanical and on the pedal cover as opposed to hydraulic. The failure shuttle block is a $350 item!!!!! Going custom would work OK but the stock system would be a pain to replicate (and pricey).

Mike!
mike!

I'm doing the dual master conversion on my 66 because I can and I think it is safer after having seen a fellow MG club member lose 1/2 of his brake master.

I will plumb one side directly to the rear and the other will go to the distribution block to supply the front brakes. My friend, Chris Gross, a vintage Bugeye racer, has the exact same setup (from whom I got the idea) and it works well. You'll need to block off one of the outlets on the distribution block.

I bought a coil of braking tube from Automec to do the custom routing.
Mike MaGee

I'd go for the dual cylinder. Like Les says, in the event of a failure of either the front or rear system the remaining braking is marginal, but the alternative is 'none!'. The cylinder should bolt on, but check the pushrod length, probably different from the single cylinder.
Bill Young

My car will be a V8 and I have some 4-pot calipers from a Leyland Princess for it. They have two inlets each. So I thought I would arrange the pipework so that each caliper received a feed from both circuits, and then one circuit can feed the back as well. That will work won't it? The alternative, if sticking to single circuit, is to join the twin inlets together.

Mike
Mike Howlett

Another solution...
Regards.
http://membres.lycos.fr/mgcontact/fileupload/uploads/brake.jpg
michel


After viewing 'Another solution' I can't help thinking about what Les stated. This system looks very complicated

"I am not sure I would trade from the simplicity of the single circuit master cylinder (which some feel is more reliable) to the increased complexity of the dual circuit MC unless I was required to." Les
Dana Wilson

"Another Solution" is a typical installation for racing cars where the front to rear brake bias is adjustable. It would work equally well as a 'dual' type master cylinder. As far as using one output line from a dual cylinder for each inlet of a caliper I would be wary. Any variation in pressure between the two lines would result in uneven application of the pads to the disc. I'm not sure but most dual cylinders have pressure check valves in the output side for the rear drum brakes and not for the fronts which could cause drag on the front.
Bill Young

continuation...
regards.
http://membres.lycos.fr/mgcontact/fileupload/uploads/1088618396_rearbrake.jpg
http://membres.lycos.fr/mgcontact/fileupload/uploads/1088618396_studs.jpg
michel

This thread was discussed between 29/06/2004 and 30/06/2004

MG MGB Technical index

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