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corsa 1.2 - MOT failure brake application imbalance - gooners

I'm wondering if anyone can make some suggestions as I'm a bit stumped with this one. My daughter's corsa failed its mot on ' 3.7.A.5C rear brake application uneven'. The readings given were:

Front 300 300

Rear 90 160

Handbrake 120 140

I first stripped down and cleaned the rear brake and changed the slave cylinder as it was a bit sticky. This has usually done the trick when I've had this in the past. But it made no difference, I got exactly the same on the retest. The MOT guy suggested that they do have cars coming in with pressure valve problems. This car has no ABS but has a pressure valve on each rear brake line. I replaced the one for the nearside that had the low reading but again this made no difference on a retest. I had wanted to change both valves but Vauxhall have 8 on back order at the factory and might be 2 weeks before they get one. Local motor factors say it's a dealer only part. I'm even struggling to find a breakers that might have one (though I'd rather put a new one on to be honest).

I don't think it's the shoes as they were new on last year, and I get a bigger reading with the handbrake operation.

I've read that you can get collapsed flexible hoses, but these seem to cause brake seizure rather than low readings. The brakes bleed OK and the pedal is hard and not spongy.

Has anyone any other suggestions as to what might be wrong? There isn't anything else that can be replaced really that I can see.

 

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