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MG MGB Technical - Servo inactive

It seems my servo is not producing vacuum (or is it pressure) to activate my brakes, as when I apply them I need to produce considereable force on the brake pedal in order to make them effective. The pads and linings are ok as they are working but, as I say, only with considerable "welly" do I get any stopping power.
Any advice out there particulary first checks to go for.
si robathan

The servo utilises vacuum to enhance the pressure applied within the hydraulic system. To check whether yours is working, press the brakes a few times with the engine running and remember what they feel like. Switch off the engine and press the pedal some more. If the servo is working, the first press won't feel any different as the servo should hold the vacuum. Then as the vacuum bleeds away the pedal should start to feel different, probably firmer with less free play. If you can't tell any difference, then the servo isn't doing it's job. Have a read at Paul Hunt's excellent web site. http://www.mgb-stuff.org.uk/braketext.htm#servo

The servo on the MGB is not very powerful and many owners think the brakes are better without it. For years the car was made without a servo, and when they fitted one the brakes were not altered in any way, so they should work well both with and without the servo.

You don't say what year your car is. I am assuming you have the remote servo fitted on the passenger's side of the car. If yours is a late model with the servo behind the brake master cylinder, then it will give more assistance than the remote one, so loss of it will be more noticeable.
Mike Howlett

Thanks Mike,
My model is GT 1978 and so the servo is fitted on the drivers side.

I shall pursue your comments (once the weather changes!!)

If I fit a new vacuum hose would there be any problems to look out for, as looking at the existing hose it has seen better days and may be not up to the job.

Stuart
si robathan

Stuart,
Strangely this very problem came up only a few weeks ago on this BBS. Scroll down the list to find "Bendix Brake Servo Problem" and read the posts there. Moss, like me, rebuilt his servo using a kit of parts. Mine works well now, and so I believe does his.
Mike Howlett

Cars with the integral servo and master as used from Sept 76 in the UK do get significant servo assistance and loss of the servo can result in very heavy brakes. It's earlier cars with the remote servo that only add light assistance, as the servo was optional and as Mike says the remainder of the braking system was unaltered when it was fitted.

If the engine idle speed changes as the brakes are applied and released then there is probably a split in the diaphragm or stuck valving in the servo. If there is no change in idle then either the vacuum port or hose from the inlet manifold is blocked (remove the hose from the servo and see if that makes a difference to the idle) the diaphragm and push-rod could be siezed in some way, or again there could be servo valving problems.
PaulH Solihull

Hi Si, been a bit too busy to notice your thread. Jep, repaired my Bendix servo with great success. The problem with mine was the rubber diaphram was spilt and some type of oily substance inside. This had reduced the braking ability and idle speed. After the repair amazing difference. Read my thread if more info required. The repair kit is not expensive. The hardest part was taking out the servo from the car, and seperating the two halves - had to make two flat bars to bolt on so I could use enough force to twist one half off whilst the other was grasped in the vice. It worked! If you decide to seperate the two halves be careful not to bend the bolts when twisting - I was unfortunate - however, thankfully, only one bolt was slightly bent - but I was able to 'bend' it back again.
/Moss
Moss

Hi Si, been a bit too busy to notice your thread. Jep, repaired my Bendix servo with great success. The problem with mine was the rubber diaphram was spilt and some type of oily substance inside. This had reduced the braking ability and idle speed. After the repair amazing difference. Read my thread if more info required. The repair kit is not expensive. The hardest part was taking out the servo from the car, and seperating the two halves - had to make two flat bars to bolt on so I could use enough force to twist one half off whilst the other was grasped in the vice. It worked! If you decide to seperate the two halves be careful not to bend the bolts when twisting - I was unfortunate - however, thankfully, only one bolt was slightly bent - but I was able to 'bend' it back again. Mike was kind enough to upload a plan to gain an overview of how it works - very helpful.
/Moss
Moss

It's easier if you leave the base in the vehicle and twist off the top of the housing. There is no chance of twisting the four mount bolts in the base. If you tighten the lever down onto the two front bolts with the fixing nuts, they won't bend either!
Pat Gregory

I tried with the base in the car - no room for leverage on a left hand drive and I'm left handed (yes it does make a difference) and the brake pipes and the Weber car are in the way - seperating the master cylinder to clear the push rod to remove front half is very difficult - much easier to remove the complete unit...
/moss
Moss

This thread was discussed between 01/12/2010 and 10/12/2010

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.