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MG MGB Technical - Sticking Servo?

I have recently replaced both front callipers and one rear cylinder on my '74 BGT. I have struggled to achieve a firm pedal by bleeding although on the second or third pump it is fine. I've tried all the methods in the archive to no avail. In desperation I took it to the local garage who tried with a pressure bleeder and got the same result. They then clamped the (new) hoses to isolate the system and pronounced a new m/c was needed. I've now fitted said master cylinder with no change. When trying the brakes this evening I noticed the brake lights were staying on for a second after I released the pedal. Is this normal or does it indicate a problem, ie pressure not being released in the system. The only part of the system not replaced is the servo although I removed the air filter to check the piston below it. This appeared ok but I now think it may be sticking which is why the first pump on the brakes has no pressure. Does this sound likely? or if not has anyone got any other suggestions.

Many Thanks,

Dave
Dave

A thought that has just occurred to me is that I may have re-inserted the servo piston incorrectly, causing it to jam. Can anyone tell me which way it should go in? I have inserted it pointed end first so the hollow end cups the stalk from the rubber diaphragm. This seems the logical way to do it but I'm clutching at straws here.

Again, thanks in advance for any suggestions.

Cheers,
Dave.
Dave

Your problem with an apparent sticking servo is well known and due to the fact that the servo can be difficult to bleed. The mounting of the unit in the MG is not really correct as the air valve should be on top and it actuallly faces the front at 45 degrees. The outlet end should also be higher meaning that the unit slopes up towards that end. Having said that the problem is still not really solved.

The problem lies in the fact that an air bubble becomes trapped underneath the small piston located under the air valve and this must be eliminated. Remove the air valve and then slacken off the three screws in its base until they are on their last threads. Get someone to press the brake pedal very gently by hand until the piston lifts the valve base and then remove the screws and base. Lift out the piston (unlikely that fluid will follow)and make sure firstly that it is clean. Squirt a bead of silicone grease into the bore and then replace all components. Bleed the brakes out of one wheel only.

Your brakes should now be fine. I have experienced this exact problem and the above method is the solution.

Iain
Iain MacKintosh

Thanks Iain, I'll give it another go later. Do you recall which way the piston goes in just to put my mind at rest?

Cheers,

Dave
Dave

Dave,

I haven't currently got access to my manuals as I am at work however the piston should be fitted so that the lips of the cup seal face the outlet of the unit.

Cheers
Iain MacKintosh

I always have to use a two-step process when bleeding the front brakes on my roadster and V8 or I get the same pedal response as you describe. I use a gunsons EeziBleed initially, then an assistant standing on the pedal while I rapidly open and close each caliper nipple. This *always* gets an extra 'lump' of air out, and the pedal is always fine immediately after.

I have also had the sticking-on you describe on two cars (roadster and someone elses V8) and it is the servo. Usually happens in warmer weather, mine really hasn't got frequent enough for me to do anything about yet and I can usually release it by tapping the brake pedal a couple of times. But on a friends V8 it got so bad on a tour we resorted to removing the vacuum hose from the serve and plugging it with an old spark plug. 'Cured' the problem, and made surprisingly little difference to braking effort, but be aware it is an MOT failure to have a servo fitted but not functioning. The problem is said to be in the air valve assembly.
Paul Hunt

This thread was discussed between 21/01/2004 and 22/01/2004

MG MGB Technical index

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