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MG MGB Technical - How to test a brake servo ?

Hello everyone,
I think I have a 'dodgy' brake servo. It's fitted to a MGB V8 roadster (conversion) with AP 4 pot calipers, and while the car passes the MOT with no problems, the brakes just dont seem that good. The calipers seem ok, the pads are square in the calipers with all the pistons moving, no pulling or any other problems associated with sticking brakes.
It's a 1970 car with remote servo. However when I bought the car the servo wasn't plumbed in and the ports were open to the elements. Maybe there's a clue there.
On other cars if you start the engine with your foot hard on the brake pedal it moves down just a bit, indicating a functioning servo, is this still true with the MG remote servo ?

Thanks
Colin M........
C E Muir

Colin, you are right, if the pedal does not drop when the engine is started, then you have a problem. I would check that you are getting vacuum to the servo. If you get some vacuum then you can repair the servo, but it might be easier to replace the unit.
regards John
john wright

Thanks for confirming that John. I've just tested an there's no movement. I don't think it's ever worked!
C E Muir

You can also adapt a Mity-Vac tool onto the servo's
vacuum line.

Pump out some air - and watch for any leak-down
(needle movement) on the vacuum guage that is
on the Mity-Vac tool.
Daniel Wong

Thanks for that Daniel, a vaccuum gauge for future use might be a good idea.
C E Muir

Colin,
You mentioned the servo not being plumbed in and ports being exposed when you bought the car.

This suggests to me the problem may well be internal corrosion necessitating work by a brake specialist.

I have done this myself and it was a nightmare so I would strongly suggest a brake repair place take a look.

Pete.
Peter Thomas

I wouldn't spend too much money on it if I were you, the remote servo gives very little assistance. We disconnected a friends as it had severe sticking on which these suffer from. Together we drove it more than enough to empty any vacuum left in the servo and both of us were amazed how little extra pressure was required. It was an optional extra on some cars anyway. The later integral servo is a different kettle of fish though, when that packs up the difference is significant.

If you are comparing it with modern cars then they are heavy, even with the remote servo connected. Personally I find modern cars far too light and sensitive.

The bottom line is that you should always be able to lock the wheels on dry tarmac with *progressive* braking. If you can't then there is something wrong with the brakes which could be very hard competion pads, worn discs, faulty flex hoses (although this usually causes pulling to one side), or you just aren't pressing hard enough. I say 'progressive' because you *will* be able to lock the wheels more easily if you suddenly bang them on, as there is insufficient time for the weight to transfer forwards which increases the friction between tyre and road hence needing greater pressure, and giving greater retardation, before they do lock. But lock they very definitely should.
Paul Hunt 2

This thread was discussed between 29/03/2007 and 31/03/2007

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