Our 1996 Micra failed its MOT with poor near side rear brake pressure. Leaking cylinder replaced at reasonable £13. Still failed with faulty load sensing valve diagonosed (stops rear wheels locking up). Dealer wants THREE HUNDRED AND SIXTY quid for a new valve!!
Any ideas - can the old one be 'freed' up, bypassed, or otherwise made to work??
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Are you sure it's a load sensing valve ? If it is there will be a mechanical link between the valve (usually mounted on the vehicle body) and the rear axle. This sort of setup is used on commercial vehicles. Cars normally have a valve in the brake cicuit that limits the hydraulic pressure to the rear brakes, but it isn't usually load sensitive. Testers do run into difficulties deciding about braking efforts on rear wheels, and a lot of problems are caused by unfamiliarity with the car being tested. If you can provide a bit more info. I might be able to help a bit more.
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Thanks for your help,
The tester says that the brake efficiency on that wheel (nearside)is only 1/10th of the offside. The hand brake works ok so I know its not a sticking shoe problem. I removed the drum and got my wife to press the brake pedal. The (new) cylinder pistons hardly moved - I expected them to try to pop out of the cylinder.
The car does have a mechanical set up as you describe, linked to the rear suspension (by a small coil spring) with fluid pipes leading in and out of the \'valve\'. The main dealer refers to it as a \"valve-load sensing\" but I guess it fulfils the pressure limiting function you describe.
I\'m happy there is no air in the system, so I\'m tempted to remove and try to repair the valve but any thoughts on altenatives are more than welcome.
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Yes, it is a true 'load sensing valve' - the little arm connects to the suspension and as the load on the rear end varies the valve piston moves in and out to restrict the pressure to the rear wheels and help prevent locking. These have become obsolete with the advent of ABS.
Normally what happens is that the car is seldom used with any heavy loads in the back and so the piston siezes in the 'minimum pressure' position.
You might be able to free it with a bit of judicious pressure (I was once able to do this on an Alfa). You could remove the valve and do it on the bench.
You may find that the brake unions have siezed and so you might need to replace a couple of pipes at the same time.
The other alternative is a s/hand item from a breakers yard.
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PS - I meant 'judicious pressure' as in applying an external force to the piston or maybe tapping it gently to free it.
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I used to have a Maestro van with the same problem. I used to free it up by applying heat from a blow torch to the valve and spraying wd40 on it. Basically then waggling the metal spring which went into it plus extra heat & wd 40 would free it off. It happened every year at MOT test time.
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If this is the type of valve I think it is, there should be two pistons in the valve - one for each wheel.
If you watch the valve while somone presses brake pedal, the tips of the pistons move out until they touch the spring controlled lever, deflecting it against the spring a little.
As Apprilia says, it should be possible to free off the sticking piston. If, however, freeing the piston off makes it leak, then you need to source a replacement valve.
Once these valves have been disturbed, they should be reset. The approved method for this (I imagine) will be quite involved, and will need the use of a pressure gauge, and possibly some weights in the car.
Having said that, I have never seen the full procedure being followed, and would be interested to hear how other BR contributors set these valves up.
number_cruncher
p.s. The method I use is quite imprecise, and is based on trial and error. I don't want to set it out here yet, as other BR contributors may have a better method.
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Thanks all - those tips are very useful. I now have something to try before replacing the valve: £360 is a big incentive!!
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JOB DONE !
The valve is as desribed and accessible from under the car. Checked the two pistons were moving freely (they were) and returned to MOT station. Second diagnosis focused on the n/s shoes themselves. Replaced for £21 and passed MOT.
Thanks to all for the posts.
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