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MG MGB Technical - 72 MGB Brake Servo Bleeding (Continued)

Apologies on at least two counts, first for taking so long to continue this old thread and for not getting around to finding out what the fault was before now. Work committments I'm afraid. I was unsure of how to continue a thread that is so old it is now in the archive section so have started this new one.
My problem was in trying to bleed the brakes after rebuilding the master cylinder. No matter what I tried I could not remove all the air from the system even after fitting a brand new master cylinder.
The fix was in disconnecting then bypassing the brake servo. I am not sure what is wrong with the servo or if it can be rebuilt ( I have the single circuit system where the servo sits in front of the passenger side firewall). Should I block off the connection where the vacumn line from the servo conects to the exhaust vmanifold?
Thanks to all that offered suggestions on the best way to bleed the system. One very effective method that wasn't mentioned here but was given by a local mechanic was the gravity system. All you do is open each bleed nipple in turn and let the fluid run out by gravity. Do each corner one at a time and remember to keep the reservoir full. There is no mess if you place a container underneath to catch the fluid. If you do get brake fluid on the caliper or drum backplate spray some brake cleaner and wipe off with a clean rag. This is a method you can do single handed as you don't need someone to pump the peddle for you. Using this method gave me a perfect peddle.
Thanks again for all help.
W R Jennings

The servo vacuum line cones from the inlet manifold, not the exhaust. It will make no difference to the brakes, but if the diaphragm in the servo is leaking it will upset the mixture. If you aren't using or going to fix the servo you might as well completely remove it, its hose, and seal the port (which contains a one-way valve) on the inlet manifold.
Paul Hunt

Sorry about the exhaust/inlet mixup my fault for writing my post so late at night. I would like to repair the servo as my wife drives the car regularily and would benefit from the modest boost that the servo provides. Can you point me towards a resource for info on how to troubleshoot it or rebuild it? My Haynes manual is useless on this point. I was intending on using the car with only the brake lines disconnected from the servo. Should I block off the port until I can repair and reconnect the servo? The only thing I am certain about regarding the servo is that when the brake lines where connected to it I could not bleed all of the air out of the system. Would that fault be related to the diaphram?
W R Jennings

As you say, Haynes doesn't mention it as it is a bit 'technical'. The Leyland Workshop Manual does cover it, and there are rebuild kits available. But that is when there is a split diaphragm, or other problem with the servo. I don't know what would cause the servo being in-circuit to prevent satisfactory bleeding unless the through-passage of fluid is blocked somehow, which the rebuild kit may not cover. Has your wife tried it without the servo? It's assistance is indeed very modest, so much so that when I tried one without I couldn't really tell the difference - unlike the later integral master and servo which does give significant assistance and is very heavy without.

Unless the servo is leaking and causing fluid to be sucked into the inlet manifold (which is a known potential issue with these servos) or causing the revs to alter as you apply the brake pedal, then there is little to be gained by disconnecting the vacuum hose if you are intending to repair the servo. If you remove it for repair and continue to drive the car in the meantime then simply blocking up the end of the vacuum hose will do, the ceramic end of an old spark plug and a Jubilee or worm drive clip will work fine. BT, DT, when a friends servo started locking up so badly on a tour of Ireland that we had to disconnect it, which is when I had the opportunity to drive one without.
Paul Hunt

Paul, To be honest I find the pedal much heavier without the servo than I remember the brakes being with it working. However it has been a couple of months since I drove it with servo so possibly my memory is failing. My wife is going to drive the car for the first time tomorrow so will see how she does. Can you possibly steer me towards a reference to servicing or repairing these servo's please. I am assuming the place to start is to remove it to check if it is full of fluid (I did not have white smoke out of the exhaust or any revs change when I applied the pedal however) then to remove the screws from the disc that I assume holds the diaphram. You also mentioned that rebuild kits are available, who from? Any advice or info greatly appreciated.
W R Jennings

The usual failure mode of the remote servo sucks fluid out of the master, causing the pedal to suddenly go to the floor when the critical level is passed. The only repair reference I am aware of is the Leyland Workshop Manual (possibly also the Bentley manual in the US which is a reprint of the Leyland).

FWIW the later integral servo is covered here -

For repair kist I just Googled 'mgb servo kit' and got a number of hits that look promising -
Paul Hunt

Bill, any local break repairer/dealer should be able to provide a changeover reconditioned booster for about $400.Why not give DBA a ring ? I think you'll find the repair kit is not cheap.Barrie E
Barrie Egerton

Whilst A$400 at 165 is reasonable for a replacement servo (in fact a complete kit) from Moss the repair kit is a lot less than that at 67, and Moss aren't the cheapest by any means.
Paul Hunt

This thread was discussed between 22/10/2008 and 01/11/2008

MG MGB Technical index

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