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MG MGB Technical - Airy Brakes

Hi all, hoping for some pointers on what to try to get a firm brake pedal back.

I have used gunson's EZ-bleed on the B and other cars without problem until now.

I think I made things worse this time by allowing the master cylinder to run dry (through not paying attention when setting up the bleeder that time, doh!).

I get a firm brake pedal with a few pumps but this rapidly fades. I have tried Paul H's tip where the pedal is held very firmly down while the bleed nipple is 'jiggled' on and off, but with no luck.

I'm no expert on brake systems,a dn thought I'd get some thoughts from you lovely people before I start randomly changing things!

TIA, Ian.


If the pedal "rapidly" fades, you probably still have lots of air in the system especialy if the m/c ran dry. Before changing things (what?!) I'd start from scratch. I think Paul H's tip is used only when there is very little air left in the system
Michael Beswick

Thanks for your reply Michael. I shall revisit, possibly running a slightly higher pressure on the bleeder.

By 'changing things', I was thinking about what seals etc in the system might be the most likely culprits for allowing air in in the first place. I did have a small amount of air in the system before, which I've always dealt with ok in the past, only this time, my brain didn't fuction as I set up the system, hence managing to bleed the master cylinder dry...



You must close the bleed nipple at the end of each down stroke and lift the pedal with the nipple closed otherwise you can suck air into the system past the threads on the nipple. So make sure that you do this to avoid problems.
Iain MacKintosh

Thanks for your comment Ian, although using a pressure bleeder there's no time that air can get sucked in.

I have re-bled the front end with no joy (and running the bleeder at the full 20psi).

Am I missing something (for example does the servo need bleeding?- should the engine be running?), or am I likely to have a shot seal somewhere that's allowing air to enter once more?


Ian,a good way of seeing if you have worn master cylinder seals is to press gently on the pedal, if it slowly goes down the you have either worn seals or a leak. Another check is to beg, borrow or buy a Sykes Pickavant brake hose clamp ( they are blue nylon). If you clamp off the rear brakes at the flexi obviously the try pressing the brakes and see if there is any difference, this will tell you if you have any air in the rears. Do the same at the front, doing one side at a time and do the same checks. Don't press very hard as they are not man enough to completely stop any fluid leaking by.
Hope this helps John
john wright

The servo should bleed without any difficulty when bleeding the whole system but if all else fails I can see no reason why you shouldnt try to bleed with the engine running.
Iain MacKintosh

Pressure bleed with the engine running & depress the pedal rapidly to allow the booster to operate & blow the air out of the booster cyl. Hold the pedal down for a few seconds. This only applies to the remote booster.
Garth Bagnall

Just reporting back to say thanks for the suggestions on running the pressure bleeder with the engine running. I'm yet to do a test drive, but the omens look good- with a brake pedal that's firm and stays so. Hooray!

I just bled one corner, and ran the engine and did a few good pumps of the pedal too.

A top tip for the future methinks. Thanks once again, I was starting to get a bit down on the whole bleeding thing...


This thread was discussed between 26/04/2011 and 10/05/2011

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