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MG MGB Technical - Brake Servo Refurb

I think the diaphragm has gone on my brake servo. I by-passed it as time was short but have decided I prefer the feel of the servo. Add in the fact, having checked, it was a standard fitting, by a couple of months, on my car rather than an option I will have to replace or repair it.

I see there are 2 servos available from most suppliers. The OE one is about 3 times the price of the aftermarket - so it must be better (?) Given the impact of a failure I will probably err on the side of caution and go for OE.

An alternative is to get mine rebuilt, has anyone gone down that route?

Mike Dixon

They're fairly straight forward to rebuild Mike, A good man like you would breeze through it.Just lay it out in a line on the bench as you dismantle it then work back through the line as you reassemble.

willy
William Revit

Rebuild info here https://landcrabforum.co.uk/thread/393/lockheed-servo-rebuild
paulh4

Thanks you both for your confidence in me and the link. I might have a play with the old one, which is now on the bench - nothing to lose apart from an injury releasing that huge spring....

Mike Dixon

the spring isn't very strong, you can hold the two halves together by hand and get someone to undo it then just release the halves----good idea to use typex or similar to mark the parts there so you get it all lined up again as is, but no big deal really it more or less only goes back together one way.
William Revit

It has come apart OK, the spring pressure was nothing of concern to be honest. I held it down with one hand and teased out the locking tab with a pair of pliers.

The hardest bit was getting the piston out; 'gently pull' turned out to be the rod wrapped in a rubber glove held in the vice and a few judicious 'taps' with a soft faced hammer.

Chatting to Power Track, (Mentioned in the link ) who are very helpful it is the state of the bore that is key. I will get another set of eyes to look at that tomorrow
Mike Dixon

I can imagine it should be much the same as a master cylinder bore as it's doing the same thing. After having changed three (out of four) M/Cs over the decades when the seal failed in the fourth one (the spreader cracked so the seal cocked) I decided to try seal replacement first. Squinting in the bore it was mirror-finish throughout so opted for just the seal, that was in 2015 and it's been fine since.

But the push-rod and seal came out of that easily, if yours was as tight as you say it may not have been moving for a long time and the bore may be corroded/pitted - like every slave cylinder I've had to replace, no chance of seal replacement on those.
paulh4

The corrosion is at the top around the plastic thing that holds the piston in - hence the reason it did not want to come out. Might be OK as it does not go won far , I have a mechanic popping in to help me on another matter this evening and I will ask him.
Mike Dixon

' go down far' !
Mike Dixon

If it'll clean up ok then all good, If it's too corroded up/jamming, the brakes can hold on for a bit after the pedal has been released. I've had one stick on and stay there which was a bit interesting for the guy following me.
William Revit

Sticking-on is not uncommon and seems to be associated with the air-valve. My roadster will do touring at slow speed in hot weather but the V8 never has, tapping the brake pedal usually releases it. A pal's V8 was so bad touring Ireland we had to remove the hose from the servo and plug it with the pointy end of a spare spark plug. Some have said brake fluid or silicone grease on the upper part of the valve fixed theirs, others talked about polishing the bore of the air valve with fine wet and dry i.e. removing a tiny amount of metal.
paulh4

This thread was discussed between 28/03/2025 and 02/04/2025

MG MGB Technical index

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