Adding a brake servo

Technical MGB discussion
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David Witham
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Adding a brake servo

Post by David Witham » Wed Jul 29, 2020 1:19 pm

My 1974 chrome bumper MGB was originally built to US specification. As a result it was LHD with a twin circuit non servo braking system.

When I converted it to RHD I changed the brake master cylinder to a UK type single circuit one as the UK cars never had the non servo twin circuit system, the master was scored inside, the pressure failure fitting was leaking and the pipes come out the wrong side for twin circuit master to be easily fitted in the RHD position.

I am now wanting to fit a servo. The easiest thing to do would be to use the type of servo available as an option on the single circuit cars in period.

I briefly considered using the late rubber bumper system with twin circuits and a servo that acts on the pedal rod. I believe that type of servo is technically safer. However, it seems to be a lot of work. The hole in the bulkhead and pedal box are different, the fuse box would need to be moved so wires would have to be lengthened etc. If someone has actually done it I would like to hear though.

2 questions about using the single circuit servo:-

1) In the UK these servos seem to be available with either 1.65, 1.90 or 2.30 times assistance. I think the original was 1.65. Is that correct? Has anyone used the more powerful ones?
2) At some time I read that there is a risk that an internal failure in this type of servo can result in the engine sucking all the fluid out. Is this something that actually happens or is it a scare story?

Ian Fozzard
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Re: Adding a brake servo

Post by Ian Fozzard » Wed Jul 29, 2020 5:50 pm

Hi David. I can't answer all your questions but can offer the following from my experience:

My 72 BGT has the single line remote servo, always worked fine and not had any attention. I do change brake fluid regularly which may have helped it's longevity.
This kind of servo (on the opposite side of the bulkhead from the master cylinder), a remote servo, takes a line from the master cylinder so brake fluid is integral to it's operation. I too have heard of brake fluid being drawn into the engine through the vacuum pipe ( on Ford's in my experience!), but not on MGs. It is possible but only if a seal fails. It does not worry me.

You should consider that a servo doesn't actually improve the braking, just reduces the pedal pressure required. So the ratio is somewhat academic.

Hope this helps a bit?

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Peter Cresswell
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Re: Adding a brake servo

Post by Peter Cresswell » Wed Jul 29, 2020 11:32 pm

Hi David,
Fitting a remote servo is quite a popular addition to the braking system, and was offered as a factory option from February 1970. See here for some details: http://www.mgb-stuff.org.uk/braketext.htm#servo.
I would recommend fitting a genuine Lockheed one as it is an important part of the braking system once fitted! You will have to do a bit of replumbing to fit it - a new brake pipe from the master cylinder to the servo and a second from the servo back to the T-piece on the offside inner wing. You will also need a hose for the connection from the servo to the inlet manifold - which must have one of the various plugged holed fitted with a take off fitting for the pipe. The fitting instructions must be followed carefully, in particular the angle that the servo must be fitted at and the position of the white air valve. It is easy to fit however and most people fit it on the nearside of the bulkhead where the dummy plate that covers the hole for LHD cars is.
The link above should give you info about the servo ratios, but the higher the ratio the less feel the brakes will have.
As far as the servo sucking the fluid out of the system this does indeed happen but usually when the diaphragm fails. When it happens, you have no brakes! I know because it happened to me on my Mini Cooper after it's winter hibernation, and all the brake fuild was in the large cylinder of the servo. A new servo though will last for many years without trouble.
Ian is right that fitting a servo doesn't make the brakes better, it just reduces the leg effort to apply the brakes. An improvement can be gained by fitting MGB GTV8 pads which are about 15% larger pad area and they fit the MGB calipers without any problems.
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

lightning
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Re: Adding a brake servo

Post by lightning » Sat Aug 01, 2020 9:55 am

Hello Peter,
I also have a 1972 US B that was converted to English spec. The "conversation" posed a question-do you get any warning that the seal you mentioned is failing before you discover the brakes have perhaps with disastrous consequences ?

Ian Fozzard
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Re: Adding a brake servo

Post by Ian Fozzard » Sat Aug 01, 2020 4:00 pm

The only time I have ever heard of any warning of brake servo failure was on a Ford. Copious quantities of white smoke from the exhaust when brake fluid was drawn into the engine via the vacuum pipe to the inlet manifold.
Whether this might happen on any brake servo failure probably depends on the design of the servo and exactly where the failure occurs. I couldn't say whether this might happen with the MGB remote servo. I believe this could not happen on a direct acting servo - no brake fluid enters the servo unit.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

David Witham
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Re: Adding a brake servo

Post by David Witham » Sat Aug 01, 2020 8:21 pm

Thanks for your comments everyone.
David

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Peter Cresswell
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Re: Adding a brake servo

Post by Peter Cresswell » Sat Aug 01, 2020 8:40 pm

Hi Ian and David
I have been giving some more thought to what happened when the remote servo failed on my Mk1 Mini Cooper - but it was 10 years ago! The remote servo effectively has another master cylinder associated with it - its the bit sticking out from the big cylinder. The diaphragm in the cylinder has a link to operate this 'additional' master cylinder and this link has a seal on it. If this seal fails then brake fluid can be drawn out of the braking system into the cylinder, where (I think) it is retained as the cylinder has a much larger capacity than there is brake fluid in the braking system. So it might not be sucked into the engine and hence doesn't necessarily give white smoke. In my case the first I knew was at the first application of the brakes - there were none! On inspection there was no brake fluid in the master cylinder at all, and once removed it could be heard sloshing about in the servo. I replaced it with a new Lockheed unit and all was well.
The servo I fitted to the Mini Cooper was the same as that fitted to MGBs and they are fairly generic across BMC sporting cars. Incidentally, my MGB doesn't have a servo as I prefer the feel I get without one. This is despite using V8 Mintex M1144 pads. The effort to brake is quite high but I can feel when the brakes are on the point of locking the wheels and modulate the pressure accordingly.
Pete
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

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