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MG MGB Technical - Hard brake pedal/no rear brakes

I topped up the fluid in my 72B with a dual circuit system, and all of a sudden the brakes were really hard. I drove it a bit, then figured I'd bleed them - see if that changed something. Found out there wasn't any fluid to the rears.

Took the system apart from back to front, right up to the MC. I can blow air backwards into the MC from the lines, but don't get any fluid flowing from the MC back to the rears. Pedal is still rock hard with little travel, and front brakes work great.

What's going on here? Is it the splitter (H-shaped) piece on the inner wing? Or something inside the MC?

Thanks
Steve
Steve Aichele

Fluid not travelling to the rear slaves will cause a shorter and harder pedal. And did you mix silicone and non-silicone fluid? I've heard that this results in goop that blocks passages in castings. Air blown backwards could be pushing this out of the way, but as soon as fluid pressure acts in the opposite direction it could be blocking the hole again. The secondary piston in the master could also be jammed, I suppose. You need to crack the pipe joints working from the master to the rear slaves to see where fluid is no longer evident, the fault lies between there and where you last got fluid. The system will almost certainly need bleeding after this.
Paul Hunt

Is the brake light switch adjusted properly against the brake pedal? If not the piston will not return to the top and will keep the system under pressure. RAY
rjm RAY

Steve,

The 'splitter H shaped piece' is the valve and switch to warn of brake failure at one end of the car or another. It won't cause a failure as all it does is sense uneven pressure and move a shuttle back and forth to release the switch in the middle. I'm with Paul. Something is amiss with the MC or a pipe or hose before the rear brakes.

warmly,
dave
Dave Braun

If the (mechanical) brake light switch is screwed in too far that tends to cause the brakes to bind on when they get hot, not prevent fluid getting to one or more brakes, quite the opposite in fact.
Paul Hunt

Thanks for the tips. The brake light switch (and pedal box cover) is completely removed for the moment, so that's not it.

I always wondered what that H-piece was for - it's not hooked up to any wires. Suppose I should probably get that together.

This "secondary piston" you mention sounds like a likely suspect. Don't suppose anyone has a diagram or cutaway showing how it goes together?

Thanks
Steve
Steve Aichele

Paul-

I should have checked your website. Excellent diagrams, and the animations on the 'how stuff works' pages are great.

Steve
Steve Aichele

Very timely, I only put them up a few days ago :o)
Paul Hunt

This thread was discussed between 03/11/2009 and 05/11/2009

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