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MG MGB Technical - Bleeding brakes?

Hi all,

I have just rebuilt the brake master cylinder on my '79 MGB. I have reinstalled it and am attempting to bleed the system. I tried starting with the passenger side rear wheel, but when I loosened the bleeder screw & attached the hose to it, only a little fluid came out & then nothing, no matter how much I pumped the brakes. Tried the other side on the rear & same thing happened. The front bleeds fine. Before you say it, it did the same thing before I rebuilt the master cylinder when I was trying to bleed all the fluid out of the reservoir to remove it. Never could get anything to come out the rear bleeders. I had to remove the line from the MC and soak the leaking fluid up with rags before I could remove the reservoir. Any ideas? Maybe the rear line is plugged or something?
Thanks,
Rodney
Rodney Isom

Have you checked the lines running along the rear axle for crushing? If the car has been flatbedded at some point in its past, it probably was tied down with straps/chains around the rear axle.
Maybe the brake differential switch has shifted, blocking the rear.
Barring that you could remove the bleeder(s) completely to see if they're blocked.

Mike
'79B
Mike Janacek

Rodney,

Just a thought...
I had a similar problem at first with my 78 GT. I then realised that the reservoir has a wall inside which means that you have to fill it to the brim, before any fluid flows over the top of the dividing wall and starts to fill the front section, which supplies the rear brakes. So I was trying to bleed the rears, thinking I had a 1/2 full reservoir, when there was no fluid in the front section of the reservoir at all.
Also, I presume you have unscrewed the pressure switch 3 1/2 turns?
Chris
Chris Goodchild

As Mike says, could well be a crushed brake line. Along that same line, it could be the rubber hose deterioriating causing the problem. If you don't find a crushed brake line, it would be interesting to disconnect the rubber hose at the axle side, push the pedal and see if fluid streams out. If not, then disconnect the hose at the frame side, and try again. If fluid still just dribbles out, then I'd follow the metal line up the car and see if it has a dent in it somewhere.

Matt Kulka

I had the same problem, but assumed it was because I was using the Rover SD1 back axle. It turned out to be two problems

1. Flexible pipe between body and axle had kinked and was trapping fluid

2. There was a small airlock in each slave cylinder, it took a lot of pumping to clear.

good luck
Mike
Michael barnfather

So, you're supposed to unscrew the pressure switch before you bleed? I also read you're supposed to have the ignition switched on as well. Is this true? Why? Thanks.
Brian Johnson

I've seen bleeder screws so plugged with crud that nothing will come out. You might try removing them completely and cleaning them with a small twist drill...
Rob Edwards

This thread was discussed on 17/05/2002

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