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MG MGB Technical - Brake Servo

Anyone knows if you can fit a brake master assembly with build in brake servo - as on rubber bumper /US models - to a LHD chrome bumper MGB?
JPM Mortensen

I think there may be a clearance issue with the rear carburettor.

The late US models had only one carburettor.
Dave O'Neill 2

I don't have a LHD MGB, but I have done this on my 1969 UK MGB GT which is a V8 conversion. It was recently written up on the V8 Register web site. Here is the link At the end of the article there is a "More" link. Click on this and it opens the full document as a PDF file.

It isn't completely straightforward as the pedals and pedal box need to be changed for the later type and that involves enlarging the hole in the engine firewall where the pedals pass through into the cabin. I also had to hammer a dent in the inner wing to make room for the body of the servo. I hope the article helps.

I have been very pleased with the conversion as the in-line servo has no brake fluid in it, so sudden brake failure, as has been reported many times with the remote servo, is unlikely to happen. And of course you get the added advantage of dual circuits. I bought a complete pedal assembly, master cylinder and servo from Andy Jennings ( All components needed total rebuild, but once fitted they have performed perfectly for the last 9 years.
Mike Howlett

Dave posted while I was typing and he may be right about clearance on a LHD car, especially if you use the factory air cleaners. I can make some measurements if you like so that you can assess whether it might fit. What measurements would you need?
Mike Howlett

Thank you so much, both of you Gentlemen.
Obviously not many that have done it on a LHD car.
I will consider once more, if it is worth the big job.
JPM Mortensen

What is your carburetor set up? You need to measure it to the firewall to see if you have enough clearance.


79 MGB
gary hansen

I didn't think it made much difference to the brakes by adding a servo.

Adrian, you are right regarding the remote servo, but the in-line servo has a greater boost and is well suited to the car. My V8 has 4-wheel disc brakes with large 4-pot calipers on the front and the servo is good to have.
Mike Howlett

"the in-line servo has a greater boost"

Correct, but from the factory that only compensates for differences in the ratio between master and calipers/wheel cylinders, it doesn't make the brakes more powerful than the single-circuit remotely servoed system (maximum retardation is limited by the tyre contact patch anyway). The difference is that if the remote servo is not functioning the difference in braking is barely noticeable, but if the integral servo is not functioning the the braking is severely affected.

That's not an argument against dual-circuit systems, although with the MGB type they are split front and rear, and if the front circuit should fail you will have virtually no brakes, and if the rear circuit fails you probably wouldn't notice.

Single circuit brake failure reported 'many' times? Not in my membership of several MG fora over 15 or more years, no more than half a dozen I'd say. There may be less things on the integral servo that can go wrong to allow the engine to swallow brake fluid, but it can still happen, and this would affect the main front circuit.


"Single circuit brake failure reported 'many' times? Not in my membership of several MG fora over 15 or more years, no more than half a dozen I'd say."

Have a look at this:
Mike Howlett

Fair enough, I wasn't aware of that number of V8 failures, but given the ratio of V8s to 4-cylinders with remote servos I'd expect the latter to be in the hundreds, and like I say I can't recall more than half a dozen at most.

Nevertheless, I have a fluid level warning system on both mine.

With these reports of brake failures on cars fitted with indirect action servos, how many I wonder of these owners, complied with the service recommendation's, for brake system maintenance, not many, if any I would be willing to bet, if people don't maintain them correctly then this is what happens, A.T
Andy Tilney

Whether it is inline or remote - both master cylinders have fluid seals between them and the vacuum chamber which will allow fluid to be sucked into the servo and thence to the inlet manifold.

Failure of the seals is common - you just don't see it with servo assisted brakes.

In my experience these do not fail suddenly but over a period of time - signalled by fluid levels dropping.

Correct maintenance is essential !
Chris at Octarine Services

It can be done, but you will need the entire pedal box assembly, complete with pedals, from a Rubber Bumper car. However, be prepared to do some modifications to the body in order to mount the pedal box. Braking power will not increase, but pedal effort will decrease.
Stephen Strange

"from a Rubber Bumper car"

Not all of them, only from the 1975 model year if to North American spec, 1977 model year otherwise.

This thread was discussed between 05/02/2018 and 09/02/2018

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