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MG MGB Technical - Bendix Brake Servo problem

Hi all,
Idle speed tries to increase when braking - a rushing air type noise occurs. I don't think the servo is working properly - braking effort required is excessive despite new disks and pads. Could this be a problem with the non-return valve or a punctured diaphram in the servo? I have a 77B with dual line brakes and Bendix servo.
Any advice much appreciated...
/Moss
Moss

Sounds like a ruptured diaphragm. RAY
rjm RAY

Hi Ray, is it difficult to repair? - Have tried to find instructs on the net on 'how to' but no avail. One can buy the repair kit - but whats the procedure...as you can guess no experience what so ever in this matter! - but willingness to try!!
/moss
Moss

I rebuilt mine using a kit. It is a pretty simple construction. Take it to pieces carefully and note what goes where and you shouldn't go wrong. I've got an exploded diagram of the servo which is some help, but I can't remember where I got it from!
Mike Howlett

Hi Mike, did you also replace the non-return valve?
/Moss
Moss

Moss,

I'ts pretty simple, If you buy the repair kit it includes all the parts you need. the hardest task is splitting the halves. I found the kit on ebay for about 15 uks. The kit includes a diagram (which is probably where Mike got his)
Pat Gregory

Moss. The rebuild instructions, including the tooling necessary (has to be custom built), are included in the factory workshop manual for the rubber bumper MGBs. Taking the servo unit apart without the proper tooling can be a quite interesting experience. Sufficiently so that I made up the tooling to do the job properly. The rebuild kit will also have some form of instructions included, at least the kits I have purchased.

Les
Les Bengtson

Hi Les, Pat, I've got the English market dealer workshop manual (pdf, 412 pages), unfortunately the Bendix brake servo rep is not included. Anyways I'm going to buy the repair kit (53 plus postage at Sussex MG parts). The Haynes manual gives intructs on how to take the servo out of the car, so thats a start...I'll then have to work out which tools to buy/adapt to take the two halves apart - and how to put them back together again. Any advice?
Very exciting stuff...
/Moss
Moss

Moss. If I remember correctly, you have a North American specification car. Thus, a copy of the manual for the NA spec cars would be of great assistance during the period of your ownership of the car. The Bentley Publishing Company has a good copy available for a reasonable price.

The instructions for taking apart the servo unit start with "Note: Special tools should be made for the dismantling and assembly of the servo, see Fig. M.7, Fig. M.8, and Fig. M.9". These illustrations give the dimensions of the basic tools necessary to rebuild the servo properly. It may be possible to get by using "work around", field expedient tools to install the servo cover seal, but damage to the servo could easily result if you do not have the proper end cover removal tool and dummy flange.

Figure M.10 gives a full illustration of the disassembled servo unit with the parts in their proper order. Of specific interest is item 14 (retaining key) which is both hard to see, unless you know to look for it, and has a great potential to damage the valve body if not removed prior to removing the valve body.

If you are going to attempt this rebuild, you the proper tools and the proper manual.

Les
Les Bengtson

Hi Les, thanks for the advise/instructions...did not know that such a manual existed. You are quite correct I have the North American specification - although now with Weer 32/26 dgev weber and all the emision stuff removed, including air conditioning.
It is also a wise comment about doing it right! It is the brakes I am attempting to work on, so it has to be done properly.
/Moss
Moss

I didn't use any special tools at all and the job wasn't difficult as far as I remember. Here is a tip I found on splitting the halves.

You can split the two halves easily by removing the master cylinder, attaching a long drilled bar or length of dexion to the studs on the front of the chamber and GENTLY turning the unit. You will see that the two halves engage with an indexed keyway which pushes the halves together. you will need to jiggle the front in order for it to dis-engage, and remove it carefully in case the contents go everywhere under spring pressure. Possibly not orthodox, but it was almost impossible to split the unit off the car as we couldn't grip the round case securely enough. Once it's in two parts you can either work on the base on the car, or undo the four studs within the pedal box to remove it.

Although its a few years since I did mine, I seem to recall that once you have twisted it and got it apart (the internal spring pushes it apart once the lugs disengage) replacing the pieces with the new ones in the kit was straightforward.

Anyway, mine has been fine ever since.


Mike Howlett

Hi Mike, thanks for the effort with the diagram and details. Its all beginning to make sense now. Looking at the pictures looks straight forward..a,b,c..... did you replace the non-return valve at the same time?
/Moss
Moss

Moss, I simply replaced all the bits that came in the kit, and a non-return valve wasn't part of it. I kept the old bits so I have photographed them for you.


Mike Howlett

Mike, many thanks again for all the effort on your part. Great. I do not know if the non-return valve is the cause of the problem, so I'll replace the existing whilst I'm at it. At least I'll know its been done together with the servo. My situation is that I have no history about the car - I finding things out as I go along. Some stuff has been positive, some not, but then that's the charm of having an old sports car!
/Moss
Moss

Make sure it isn't the hose first!

Non-return valve won't cause the change in idle or the rushing noise (unless the casing is fractured). It will stop the servo working at all if stuck shut, or stop the servo holding a vacuum with the engine off if stuck open.
PaulH Solihull

Hi PaulH, have checked the hose - nothing wrong there, intact and tight fit at both ends - in fact looks quite new.
/Moss
Moss

Update and advice needed....Hi All
Received the repair/service kit. Now in the process of trying to work how to get the servo off and ran into a problem - cannot see how I can take the bottom two nuts off (out of 4 - two at the top easy to get to). The mechanics/parts inside the pedal box are in the way, so can't get to the nuts. Does this mean that I have to take the pedal box out with the servo attached and then disengage the servo from the pedal box out of the car?
Any comments appreciated..
/Moss
Moss

Whoops...more haste less speed...there came light...the work lamp was in the footwell, head down in the pedal box, saw the light from the footwell up into the pedal box...ah, perhaps access to the last two nuts through the footwell and lo and behold this is the case. So problem solved. The next step is getting the spil pin out of the clevis bolt on the brake mechanism. It is really twisted and bent back.
/Moss
Moss

Have the sevo out of the car and attached two bars front and back on the bolts and in my vice on the bench - seems very very stiff - is there something that has to be removed before seperating the two halves. Does not seem to be able to twist
Any suggestions?
/moss
Moss

Jep, it helped to twist the right way! However, now here is the real puzzle...as the two halves seperated fluid (feels like oil) poured out, about enough to fill an egg cup!
Where on earth did this come from? It is not petrol (no smell), cannot be brake fluid (the master cylinder seals are intact, no spillage anywhere and since getting the car brake fluid level has been the same). This oil is very thin, has a light brownish colour, and no odour.
Has the valve inside the servo fluid inside it or is it just spring loaded?

The residue from this stuff is over everything inside the servo.

Any suggestions very very welcome!
/Moss

Moss

I don't know what is inside your servo, but I DO know that it shouldn't be there. This type is dry inside. Just a smear of brake grease on the moving parts.
Mike Howlett

Indeed Mike. It should be dry. Cannot fathom it.

1. Also been thinking about how the servo operates. How can pressing the brake pedal alter the idle speed? I have a weber 32/36 dgev carb. The only connection to the inlet manifold is the vacum hose with a non return valve connected to the servo body. The rubber seals on either side of the this valve look intact.

2. Another thing; does anyone know how far the pushrod is suppossed to protrude into the master cylinder? With the repair kit are instructions on how to adjust the rod but by how much is not given. Could it be that this is incorrect and therefore not pushing far enough and hence not enough pressure building up.

?
/moss



Moss

1.I think I have worked out how the idle speed gets affected. I have taken the non-return valve out of the servo. When blowing through the end which sits in the servo air comes out the other side (i.e. through to the vacum hose end). This must mean that this air (when created by the vacum in the servo) goes down into the inlet manifold and alters the air/fuel mixture which then affects the idle.

2. A question; should air pressure be going through the otherway (i.e. the pressure created in the inlet manifold up to the non-return valve and then into the servo) thereby creating a vacum?. If not what is the point. If air pressure only goes the other way down into the inlet manifold then it can serve no purpose in the brake system because pressure is redirected and not staying in the servo!

Help!
/Moss
Moss

Moss,

I don't know the details of exactly how it all is controlled, but I think the principle is that the vacuum from the engine lowers the pressure on one side of the booster to below atmospheric, and then the atmospheric pressure on the other side helps to push the brake rod.

I suspect the fluid you found is brake fluid from a failure that occurred before you owned the car.

Charley
C R Huff

Hi Charley, I have since found out that yes it is do with pressure both sides of the rubber diaphragm inside the servo. So your're right. High pressure on the side of the brake pedal, and low pressure on the side of the master cylinder. The rubber diaphragm is in fact intact so I assume my problem, apart from the oil like fluid, is the valve in the servo . Apparantly the 'push' valve inside the servo operates thus shutting and opening after the pressure. I think!...bit of a conundrum actually. I have the repair kit so first go is this otherwise I'll have to buy a new a unit.

Regard the non-return valve this is how it should be. Pressure escaping from the servo when needed. So the issue is the valve not working correctly - that is sharing the right pressure between the two sides of the rubber diaphragm!!

Regard oil like fluid in the servo, could well be an earlier leak from the master which has since been repaired as you suggest - although how so much brake fluid can get into the servo is astounding. There are no leaks from the master or ealier signs of leaking around the area below servo and master.

So I think I know what the problem is but yet not 100% certain...
/Moss
Moss

The reason applying the brake affects the idle is that the servo is faulty!

The purpose of the one-way valve is to maintain a depression in the servo after the engine has stopped, in case the engine dies when you are moving and you take it out of gear of dip the clutch before bringing the car to a halt. There should be enough depression left for 2 or 3 applications of the brake, but the servo effect will reduce with each application. There is never any positive pressure in the inlet manifold under normal circumstances, and even if there were it wouldn't reach the servo.

Ordinarily, brakes not applied, manifold vacuum is applied to both sides of the diaphragm so it stays where it is. When you apply the brakes first one valve between the two sides of the diaphragm closes, and then if the brakes are applied harder a second valve opens the back of the diaphragm to atmosphere. With a vacuum still on the front atmospheric pressure moves the diaphragm, which pushes on a piston inside the servo, to apply additional pressure to the fluid i.e. servo assistance.

When the brakes are released first the valve open to atmosphere closes, then the valve between the two sides of the diaphragm opens, which evacuates the back of the diaphragm again, and the return spring pushes it back to release the brakes.

These valves are controlled by fluid pressure in the remote vacuum, I don't know whether it is fluid or mechanical in the integral servo.

So each time you apply and release the brakes a small pulse of air flows via the servo into the manifold, but this should *not* be enough to affect the idle. Only if the valving, or something else linked with it is faulty such that there can be a *continual* flow of air through the servo into the manifold, will the idle be affected.
PaulH Solihull

Hi PaulH,
Thanks for the description on the principles of the brake servo. Now understand how it works and why it is not operating.
Have now seperated all the parts - the valve is not operating at it should, hence change of idle speed and no servo assistance.

I'll be back with an update shortly...
/Moss

Moss

I did it, yes it is fixed, all the seals are replaced. It turned out that the rubber diaphragm was spilt at the edge - could not see it at first. I've even painted the two halves with silver metalic Hammerite after removing the old paint and rust. Looks like new....lets hope it works when back in the car. I took a few pics to upload they were too big for the site. Be a while before I find out as I've decided to take the pedal box out as well. Rust underneath this and I've discovered a few bolts missing. The engine is coming out soon...a bigger job!!

A big thanks to everyone for their contributions. It helped alot...
/Moss
Moss

After all the help with servo here is a pic of the B in gratitude...
/Moss

Moss

Interesting to note your folded hood is quite high, too high to get a cover over and fastened. Mine was like that when I got the car, but I found that a bit of judicious bending of one of the arms each side allowed it to fold right down into the space behind the seats, and allowed the fitting of a cover over it. However the only way it will do that without tearing the material is if I disconnect it from the rear deck first, which was something I didn't have to do before. One makes one's choice. Since then I've noticed any number of cars some folded high and some low, but no rhyme nor reason with the age of the car.

PaulH Solihull

Hi PaulH, you are quite right - the hood at the time was folded incorrectly for fitting of cover (pic taken in the summer - winter in DK now, had first sleet already and minus temps at night). Since then I have followed the advice of John Twist at University Motors. He's made a good clip one can see on YouTube. Very easy to do when one has learned how...and I have found out the type I have is called the double duck!! What ducks have to do with a hood is beyond me but perhaps something do with two layers of linen then rubber membrane in between....if I've understood correctly...
/moss
Moss

Cotton duck is a type of canvas - http://en.wikipedia.org/wiki/Cotton_duck
Chris at Octarine Services

Hi Chris, indeed, Wikepedia is very useful. The 'double duck', perhaps, is a metaphor for being waterproof. It certainly costs more to buy than a vinyl hood. I have no idea how old the one on my B is. It is faded but waterproof. Bought a cleaning and conditioning set in UK in the sommer. Very good.
/Moss
Moss

Also the expression "tight as a duck's a**e" may be relevant, that being water tight (or it would sink). Double-duck obviously meaning twice as water-tight.
PaulH Solihull

In fact I have found out that the dutch/afrikaan in South Africa call cotton cloth 'doek'. So perhaps a historic lingustic link for the use of 'duck' and a roadster hood...
All that because of a 'double duck' hood on a mg roadster...
/Moss
Moss

At last a chance to test the new servo - WHAT a difference! There was certainly something wrong with the 'old' servo. Now, compared to before, instant response and less effort required to brake - now front end dip and really shiny brake discs :) can be seen through the wire wheels - and the idle speed not effected anymore. Quite pleased with myself actually, my first 'difficut' renovation completed with success.
Once again thanks to all for comments and advice...
/Moss
Moss

sorry to enter this late, but does anyone know where hwo to remove the retaining key. Finally split to e halves and thought I was home free. Can't find the retaining key, manual says push valve rod to remove key.

Help, please!

Thanks
Barry
B Oyoung

Barry. It has been a while, but I remember it being between the diaphragm and the valve body, mostly hidden by the rubber of the diaphragm. Simply slides out to one side, allowing you to disassemble the internal pieces. A golden colored, rectangular shaped piece of metal.

Les
Les Bengtson

Got it, right before my eyes, thanks Les!
B Oyoung

This thread was discussed between 26/10/2010 and 02/01/2011

MG MGB Technical index

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