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MG MGB Technical - Bleeding, where is the air coming from?

Hi,

Just replaced my rear wheel cylinders. Did each one separately and reconnected the hydraulic line fairly quickly, before attempting to fit the clip. Figured that I'd only need to do a bit of bleeding.
Anyway over a litre of brake fluid later I'm still getting copious quantities of air coming out. Where is it coming from?

The car is a 66, with single circuit brakes and no servo. Apart from the rear wheel cylinder weeping, there was nothing wrong with the brakes. I am using the method, with an assistant, of open bleeder, push down, hold while the bleed screw is tightened, then up. loosen bleed screw then down and hold, etc.

Any ideas? Brake fluid is getting expensive.

Herb

PS, Just thought about air getting past the MC seals on the up stroke, with the cups collapsing some.

PPS. Please don't give me advice on bleeding techniques, as this is not my problem. It may not be the nifftiest way to bleed, but has worked for around 50 years and is cheap, except for the fluid.


Herb Adler

Herb - I would sure look at the MC seals - they're cheap to replace. Cheers - Dave
Daved DuBois

Thanks Dave,

I will pull the MC and probably get it sleeved with stainless.

Herb
Herb Adler

Hi,

Well I found the cause. Lots of pitting in the MC.

Herb

Herb Adler

Herb
Along with what you have found there is a problem bleeding the rear brakes
As you can see in your pic. the bleeder is on the bottom of the cylinder and the supply at the top
Unfortunatley gravity takes over and the air goes to the top of the wheelcylinder and fluid comes out the bleeder leaving air in there
I have found the best way to bleed them is to let the fluid flow out the bleeder by gravity without pumping till you get a good air free flow through your bleeding tube - then shut the bleeder - get your assistant to SLOWLY pump the brake up and then bleed each cylinder at the pipe fitting instead of the bleeder. This gets the air out of the top of the cylinder.
Works for me Willy


William Revit

Hi Herb, when you are bleeding the rears, adjust up both brakes fully, this cuts down the air space to about nil, that normally sorts it.
regards John
john wright

This thread was discussed between 16/09/2012 and 23/09/2012

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