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MG MGB Technical - Rear brake failure problem

My car is a 1980 B roadster UK spec. Following an MOT failure (offside rear brake recording little or no effort) I removed the slave cylinder today, and found it to be seized. I fitted a replacement and then set up the Eezibleed. I opened the bleed nipple on the new cylinder but no fluid came out. I removed the bleed nipple completely still no fluid. I closed the bleed nipple and applied the brakes but couldnt see any movement at the slave cylinder. I noticed that the brake pedal had a short and firm travel, and if I applied more force to the brake pedal there was a faint sound of air hissing at the master or servo. I had a look at the other rear wheel (which I havent done any work on) and found that it could be rotated freely when full pressure was applied to the brake pedal. Opening its bleed nipple, no fluid came out. I found that opening the bleed nipple on a front calliper produced a flow of brake fluid. So Ive completely lost hydraulic pressure from the rear brakes circuit. The master cylinder reservoir is full to the brim. Can anyone suggest what might be the problem?
Brian Shaw

The rear flexible hose can collapse. This normally stops fluid returning, thus locking the brakes on. It would also prevent the use of an Eezibleed.

However, I haven't seen one that bad that it won't let fluid through with pedal pressure.

It could be the pressure differential unit.

Have a look at Paul's 'brake' section....

http://www.mgb-stuff.org.uk/braketext.htm
Dave O'Neill2

When the flexible starts to perish, the rubber breaks down and creates a blockage. I have known them resist every effort to push fluid through!
Allan Reeling

Has the car ever been towed from the back? If so, the tow truck driver may have installed his tow chains around the top of the axle, causing the rear brake lines to collapse. This stops the fluid from reaching the rear slave cylinders and thus no rear brakes. RAY
rjm RAY

I had a problem just like yours when bleeding and it had to do with the brake switch thats screwed in behind the master cylinder. I looked into everything else. Unscrew the brake switch a little try bleeding the brakes the old fashioned way with 2 people. Thats what did it for me. Then test function of the brakes, then tighten the brake switch back up and double check function of the brakes.
JRB Mr

It's never been towed; I've done a visual check on the brake pipes underneath.

I unplugged the pressure failure switch which screws into the underside of the master cylinder, and unscrewed it 3 1/2 turns.

I might disconnect the line that feeds the rear brake circuit at the master cylinder and see if pedal pressure forces anything out of the master cylinder outlet. The front circuit is ok; operating the brake pedal locks the front wheels. Does anyone know how much brake fluid I will need to flush/refill the braking system?
Brian Shaw

Two pints, to be on the safe side. RAY
rjm RAY

After a master cylinder failure and replacement I was trying to bleed everything with a Mityvac. No success.
Replaced the flex lines front an back and everything is back to normal.
William Bourne

If you disconnect the rear flexi from the front to back pipe, can you push fluid out? Might indicate whether the problem is rear flexi related or M/C. By then you might as well replace the rear flexi anyway.....
Michael Beswick

Michael, I'll check the rear hose tomorrow (too cold today). Looks like it's either that or the master cylinder.

Re master cylinders: Moss list two for my car - GMC164 (O.E.) at £140, or GMC164Z (aftermarket) at £70. Would like to hear from anyone who's used the aftermarket version..
Brian Shaw

Might want to put the switch 90 degrees from where you have it now as well if you arent getting anything... How are you bleeding your car?

JRB Mr

JRB - I'm bleeding the brakes with Eezibleed. I've used it a few times successfully in the past, it's working this time (good flow of fluid comes out of the front caliper bleed nipples).

I assume the switch is just for 'brake failure' indication? The last time I did a brake bleed I had to unscrew the switch to unlatch it and extinguish the failure lamp.
Brian Shaw

Hissing from the servo is normal, even when the engine is off, as the servo holds the vacuum for a while.

When bleeding brakes on the later servo-assisted dual-circuit master you do have to unscrew the balance switch a couple of turns. If not it locks over towards the circuit you are bleeding and I think closes it off as well, to reduce fluid loss.

If the handbrake is adjusted correctly the slave pistons shouldn't be all the way in, so in theory you could slacken one nipple then push the other two back together, which should push fluid out. However having already changed one slave maybe not. That would prove the pipes along the axle are OK.

PaulH Solihull

Hi Brian,

When I used the 2 person method of bleeding the brakes after I adjusted the brake switch I had fluid come out in no time... I was using a system that would pull a vacuum on the line before that and wasnt working...
JRB Mr

Solved, I think. I disconnected the rear flexi hose from the feed line and the fluid came out of the line when Eezibleed was connected. I found the flexi was impossible to remove from the 3-way brass connector, and one of the lines out to the offside rear slave cylinder was seized to the connector. Had to remove the connector + flexi + flexi-to-battery tray bracket + offside line, then remove the flexi on the bench. It was impossible to blow down it. Cut through it and found it to be well blocked (I don't recall ever replacing this flexi, it's probably an original part). I have a Goodridge flexi set on the shelf. Thanks for all the comments to this thread.
Brian Shaw

I now intend to bleed the brake hydraulics. I've only been working on the rear brake hydraulics - can I bleed from just the two rear wheel cylinders? I know the fronts are on a separate line from the master cylinder which has been kept full to the brim for the duration of this work. I intend to do the whole lot anyway to ensure I've got fresh fluid throughout, but I may be stuck for time or run out of fresh fluid.
Brian Shaw

Yes.
PaulH Solihull

Paul, well put. RAY
rjm RAY

This thread was discussed between 16/01/2013 and 20/01/2013

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