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MG MGB Technical - Brake boost 1976-1980

I dunno....The braking, supposedly boosted, on the 79 and 77 seems about equivalent in foot pressure to the 1971 unboosted flavor. Any hints on tweaks or insider knowledge of how to kick up the vac act on these big round imposers? Would a smaller id hose help it suck better? Is a bearing one way valve the ticket over the plastic diaphram? I'm considering deep 6 ing them and going with Jeff Schlemmer's very clever pedal box/ lever improvement he shows on his site.
What do you all out there advise this viejo then?
Cheers, Vic
vem myers

Do you need it? On the 71, can you lock the front wheels with hard -not jammed pressure? If you are happy with that.....
My understanding is that the boost was never that high in the first place
Michael Beswick

I think a lot of the reason for using a servo was to be able to use harder, and therefore, more fade resistant, pads. If you want more initial "bite", go for softer pad material, but they will fade quicker. Some people use green stuff, I have tried them and didn't notice the difference! Do a vacuum test on the servo to see if it's working properly.
Allan Reeling

"The braking, supposedly boosted, on the 79 and 77 seems about equivalent in foot pressure to the 1971 unboosted flavor."

It probably is, there would be no reason to go for highly sensitive brakes, and the factory probably intended the end result to be much the same. I've not noticed any real difference between that, the earlier remote boosted, and the earlier unboosted. The difference is that if the later servo integral to the master fails, then you *do* know about it, or so I've been told.

Changing the hose diameter won't do anything as there is very little flow in the hose, only negative pressure (and if size *did* matter, a reduced diameter would give reduced assistance). The amount of servo assistance is a factor of the size of the diaphragm and the bore of the master.

As Michael says, if you can lock them with gradually applied pressure (not banging them on which locks easier) they are good enough. If you can't then there is either something wrong with the braking system or the amount of effort you can apply to the pedal. The latter case *would* be a reason for seeking more assistance.
PaulH Solihull

Here is a question; for those whom have V8 conversions, were you also 'forced', then, to have a brake conversion? After all Paul, I think you stated that the Rover V8 is almost the same weight as the 1800 engine. If we all abide by the road rules we will not be going to excessive speeds, then it makes sense the standard brake system should be sufficient? I am anticipating a conversion, but I know what is going to happen,'oh dear you will need to uprate the brakes, or it's no go' says the engineer! Some of the early Corvettes came from the factory, with 370BHP, and no power brakes ( late 60's early 70's. ) These servos were a factory extra! Mike
J.M. Doust

The weight of the 77-80 rubber bumper cars is somewhat more than the weight of the earlier, chrome bumper, cars. In fact, the rubber bumper roadster weights slightly more than the chrome bumper GT does. While the remote servo may well have been some form of add on device of limited effectiveness, the integrated servo is quite effective. Just remove the vacuum line and plug it, then take the car for a test drive using the non-boosted master cylinder. Considerably higher pedal forces required than when the servo is functional. A properly functioning rubber bumper system will demonstrate significantly lighter braking pedal effort as compared to a chrome bumper car. I have both. Remember, that all of the MGBs used the same rear brake shoes and the same front brake pads. Hence, the concept that the factory went through the expense of designing and developing the integrated servo/master cylinder system to allow the use of some form of different brake pad is not supported by the workshop manuals.

It would be possible, should one desire, to replace the existing brake pedal box, pedal system, and master cylinder with the 68-74 type components. But, it would be a lot of work for very little, if any, gain. And, in event of any form of accident involvement, the change from what is perceived as being a better (i.e. power boosted braking) system to an older system which it replaced might make for some interesting times.

Les Bengtson

If you drive the V8 at the same speeds as the 4-cylinder, then you don't really need more powerful brakes. The stopping power is (or should be) ultimately dependant on the tyre to road contact patch. Whilst on the face of it the wider 175 tyre of the V8 might seem to need more powerful brakes than the 155 and 165 of the original 4-cylinder cars to lock the wheels (and that's only if a wider tyre with the same weight does have more grip than a narrower tyre), the much later LE had 185 tyres and 'standard' brakes, and presumably they were 'adequate'.

It doesn't matter how powerful a vehicle is, or whether the brakes are boosted or not, the driver should still be able to lock the wheels. If the Corvette could do that then there was no problem. If they couldn't, or could only do it with the optional servo, then the phrase 'Unsafe at any speed' comes to mind.
PaulH Solihull

Some of y'all may have noticed Jeff Schlemmer's post at his Advanced Distributors web site in the USA. He modified his pedal box to extend the lever arm pushing on the master cylinder, providing more braking pressure with less foot effort...or maybe it was a shorter lever throw with more pedal travel.....
Anyway, I have the un boosted 71 and a fairly fit braking system, and find little difference between that car and either the 77 or the 79 with supposedly "boosted" systems. I guess my question would be: Is it reasonable to think the diaphrams, like a ZS carb, crack and leak with age?
Probably, eh?

I'll try putting 5 # to the hose and see if it leaks down. I've tried DC'ing and plugging the hose and notice no difference whatsoever in braking with the hose connected or disconnected.
Further comment is greatly appreciated. Cheers, Vem
vem myers

Paul's quite right. Tyre/road contact and pedal pressure are what affects braking performance at town speeds and, up to a point, the size of the pads. BUT braking from motorway speeds brings brake fade (require more and more pressure as they heat up) into the equation. Braking is only converting one form of energy to another. i.e., Kinetic to heat. How well a system slows you from higher speeds is a product of how well it dissipates the generated heat. That's a product of pad material, the disc, hub and wheel and airflow. Plus how well the generated gases can be removed from the pad/disc interface. So we get, harder high tech pad material, (usually needing more pedal pressure) bigger and ventilated discs, cross drilled or grooved discs, finned drums, ventilated wheels, alloy wheels. Up to a point the heat helps the disc brakes by "self servo-ing". i.e. the discs expand and apply force to the pads. Incidently this is one of the reasons why discs are superior to drums, drums expand away from the shoes.
So, town driving it's hard to get much of an improvement really, you could fit V8 pads, they fit and are slightly bigger. Softer pads, if you can get them, will give better initial bite but will fade , from higher speeds. If you want to improve high speed performance go for V8 calipers and discs (thicker) the latter preferably cross drilled or grooved. Or V8 discs and Princess 4 Pot calipers............bigger pads still! But as Paul said this is a waste of effort unless you have decent, newish tyres.
Allan Reeling

In my opinion, the only reason that a boosted system came into being was to deal with the extra effort required by the tandem master cylinder, introduced in '68. My '67 stops on a dime, using the single line master cylinder that was standard from '62 to '67. Drivers have gotten used to being able to stop their cars with almost no pedal pressure. The braking system, on my '87 Chevy pickup truck, is so touchy that is difficult not to lock up the brakes under mild braking. Yes, the early '60s Corvettes had no booster, but up until '65 they were still using drum brakes all around. Even in '65, when disc brakes became standard, a booster was a part of the system. The disc brakes became standard, but drum brakes could still be had as a delete option. RAY
rjm RAY

"the only reason that a boosted system came into being was to deal with the extra effort required by the tandem master cylinder"

Not so. The remote servo was used with single circuit brakes from 1970 to the start of the 1977 model year, and the unboosted dual-circuit system was in use from 1968 until the start of the 75 model year.

You may well not notice much difference between the unboosted single-circuit and the dual-circuit boosted system - when the servo is working. But unlike the earlier remote servo which only gives very light assistance(actually very light, and even lighter on an earlier version), the dual-circuit servo gives much more assistance and you will certainly notice the difference if it is *not* working. Apparently. The end result of all three types, when everything is working, is very similar.
PaulH Solihull

Peace Brothers- Paul please expand some with more straight dope on the "servo that works". I've got a brand new system, complete frontend rebuild, dropped swivels with needles in trunnion, new rotors, new calipers, and green pads. All new brake lines hard and soft. New rear drums, wheel cylinders and shoes. Pressure bled a couple times and adjusted pretty darn persnickety. New master brake cylinder ( Moss Gold Classic Series). THe only original, old, used article in the system is the servo. Frankly for the dolores thrown down, I expact a bit more...... Cheers, Vic

ps- So Paul, how's the SC and EDIS doing you pioneer you?
vem myers

Vem, depends what you expect!! Give the pads and disc chance to properly familiarise themselves with one another. If you're then expecting to breath on the pedal and stand the car on it's won't. It's always going to take a shove! If there was room you might get more effect by lengthening the brake pedal!!!!! But then a longer push!!!! Alternatively get a vacuum pump off a diesel and plumb that in!!####!!! But I have to say I like brakes that don't act like a switch, but don't fade. That way you can feel the braking and have confidence that they will still pull you up after that mountain descent!!!!!
Allan Reeling

"servo that works"

All I meant by that is a comparison between all three types when things are as they should be. You will barely notice a non-functioning remote servo, but from what I've heard you *will* notice a non-functioning integral servo.

"So Paul, how's the SC and EDIS doing"

As the only Paul having contributed to this thread I assume this was aimed at me, but I'm not doing anything with SC or EDIS.
PaulH Solihull

Although this deviation from the original question may merit a thread on ots own, I refer back to my query; If one is building a 'special' ( for want of better words) Can one quote physics to the engineer when he inspects, and decides that the 'braking system' is not man enough? I agree entirely with all that has been said, but who or what body then decides that a system does not comply? Who decided for example that the V8 MGB would need thicker discs? Mike
J.M. Doust

I would have thought an "inspection" would include a brake test; i.e., a piece of engineering kit. No argument with results. I guess BMC/MG worked on the basis of, "it's a faster vehicle so we better beef up to cope with the extra 20+mph and the extra heat generated, then drive it and put it on a brake tester and see!!!
Allan Reeling

Vehicle weight and speed should say what the brakes will have to deal with. Piston size, pad area, disc diameter/thickness/design and tyre width how well theoretically they will deal with it. An engineer designing a system, or one inspecting it for approval, should be able to deal with those. But if you are building a special, and it has to pass an inspection, then I'd be asking the body that will be inspecting it what standards they assess to.
PaulH Solihull

Paul- I'm sorry, my bad. I did confuse you with an English gentleman of the same name who has pioneered some innovations on the MGB supercharger and the adapted Ford EDIS ignition. I meant no offense. Vic
vem myers

I'm not sure whether it would apply to a "special" but the UK MOT standard for both service and hand brake are expressed as a percentage of the vehicle's weight. I THINK it's min 50% for service brake (the machine calculates it, and I can't remember when I last had one fail) it's min 25% for the hand brake if single line brakes and min 16% for handbrake for dual line brakes. I'd guess this would form the basis for testing a special.
Michael Beswick

Gentlemen. The late model, integral servo braking system has decidedly reduced braking effort (pedal effort) when the servo system is in operation. If Vic finds no difference between the feel of the brakes when the servo unit is connected and disconnected, the servo is not functioning in the intended manner. Period.

With a non-functioning integral servo, the braking effort is somewhat greater than that required of a dual master cylinder system without servo (e.g. my 68 GT). The only cure for an old servo is to remove it from the system, rebuild the servo, and reinstall the servo. Then, inspect the entire system for proper functioning. But, rebuilding the servo is the first requirement for any further trouble shooting. And, the tooling needed for the rebuild is not commercially available, nor was it when the vehicle was in production. Illustrations and dimensions of the tools which have to be built are in the factory workshop manual.

Les Bengtson

Vic - none taken.

"If Vic finds no difference between the feel of the brakes when the servo unit is connected and disconnected, the servo is not functioning in the intended manner."

I don't think Vic is saying that. To me he is saying that the later boosted dual circuit version gives much the same feel as the earlier unboosted version, which I don't think is unreasonable. It's disabling the later servo that I think would make a big difference.
PaulH Solihull

Paul. Vic's comment: "I've tried DC'ing and plugging the hose and notice no difference whatsoever in braking with the hose connected or disconnected" would, to me, indicate that, yes, he has tested the system with the servo connected and disconnected and found no difference.

His second statement: "THe only original, old, used article in the system is the servo." might be why he has been able to determine no difference between the braking effort required with the line from the servo to the manifold connected or disconnected.

Again, two things from someone who actually owns examples of the vehicles under discussion and who has rebuilt servos and retains the capability of doing so:

1. There is a large, quite noticeable difference, between the required braking effort when the integral servo is disconnected vice when it is connected--if the servo is in proper working order. If you do not feel this large difference, the servo is not functioning as intended.

2. The difference between the integral servo braking system and the earlier tandem master cylinder system is notable in side by side comparisons. With the servo in functioning condition, the integral servo system requires significantly lighter pedal pressure for the same braking force at the wheels. With the integral servo not functioning, the side by side comparison of the two systems will favor the earlier system over the later system.

So, yes, Vic's comments seem to indicate that he has not bothered to rebuild the aged integral servo and that all of his tests indicate that the servo mechanism is non-functional. Until he decides to either rebuild the servo, or to replace it with a new one, nothing more can be determined.

Les Bengtson

This thread was discussed between 26/01/2012 and 31/01/2012

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