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 Posted: Jul 25, 2016 09:14PM
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CA
In my limited experience, it is not a Brake Bias Valve failure that causes rear brakes to lock up, it is the bore size of the rear wheel cylinders.

The Bias Valve is a very simple item...it has a ball, an incline plane and a shuttle valve...on hard braking, the ball rolls up the ramp and presses on the shuttle valve reducing the hydraulic pressure to the rear wheels.

If Cooper S or Metro disc brakes have been added up front then a maximum 0.625" or 5/8" bore is mandated for the rear wheel cylinders...there were 7/16" rear wheel cylinders too in the early days.

NB - the only thing missing in the photo is the ball.

 Posted: Jul 24, 2016 06:05PM
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Nope, that would not be a symptom. Spongy pedal would be air in the system or brakes out of a adjustment.

 Posted: Jul 24, 2016 05:11PM
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Thank you all for the input! Read on some other forums that it could cause a spongy pedal, but that doesn't seem likely to me. 

 Posted: Jul 18, 2016 05:09PM
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US
I had the rear brakes staying  on, and replaced the valve. It was like the fluid could not return to the M/C. 

 Posted: Jul 17, 2016 12:14PM
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Yes, I have replaced the later style bulkhead valve on several cars. Same symptoms as Cup Cake has described. The rear wheels will lock under heavy braking which they should not do if the valve is operating correctly.

 Posted: Jul 17, 2016 04:21AM
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I had one fail on my former MkI Cooper S. The rear brakes locked up causing me to loop 180 degrees when I attempted a panic stop for an amber traffic light. The light was at the bottom of a hill and on a slight curve. It was disconcerting to say the least but nobody else was present so no damage was done. There was nothing obviously wrong with the bias valve but I did change the rear shoes. When it happened again at the same light I replaced the valve. 

The power of accurate observation is commonly called cynicism by those who have not got it. G.B.S. Sarcasm is the lowest form of wit. Oscar Wilde

//www.cupcakecooper.ca/

 Posted: Jul 16, 2016 09:19AM
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Has anyone here actually had a bias valve (proportioning valve on 76-up minis) fail? If so, what were the symptoms? Has anyone here eliminated one or replaced it with a more efficient proportioning system?