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Rear brake pressure reduction valve info

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Old May 16th, 2006, 22:51   #1
peteS40
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Default Rear brake pressure reduction valve info

Hi,

I won't go into details about why, but slight, slight possibility my brake problems could be affected by the valve which is supposed to reduce brake pressure to the rear wheels under heavy braking.

Some questions on this:

- is there any way to tell conclusively that it has failed (e.g because when it fails it leaks fluid or something)?
- is it realistic that it might have failed in such a way to constantly reduce the pressure?
- or would it just basically have no affect if it failed, i.e as if it was not there so always normal pressure to rear under any circumstances?
- any idea on replacement cost: are we talking silly money?

No time to do lengthy update on on-going brake problems etc. as forum going down in a few minutes!!!!

Cheers,
Pete
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Old May 17th, 2006, 10:28   #2
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Pete,

I think that there is a pressure relief valve somewhere though how you'd test it I've no idea - assuming there is a way. My Mondeo got recalled for a replacement of exactly this item and I didn't notice any difference afterwards.

It might be worthwhile seeking out a brake specialist and asking a few questions.......
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Old May 18th, 2006, 21:53   #3
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They don't have one so it carn't go wrong, got abs instead
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Old May 19th, 2006, 02:27   #4
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Quote:
Originally Posted by Rooster
They don't have one so it carn't go wrong, got abs instead
ah, if only that were true then it couldn't go wrong ... but no there is one left hand side of engine bay on bulkhead....2 feed lines from abs go into it, 1 for each rear brake, 2 out of it to rear brakes... p/n 30818013, "valve".... although I am more used to vw systems where it is integral with ABS unit hence some of the questions...

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Old May 19th, 2006, 18:42   #5
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Why do you think its bust,

I thought that was just a joint so if you were replacing brake pipes you didn't have to run them all way to abs unit
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Old May 20th, 2006, 03:13   #6
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According to haynes it is a pressure reduction valve like old cars would've had which reduces pressure to the rears under heavy braking to prevent the rears locking and keep the bias to the front (where all the effective load/weight is).

I should stress that for my problem it is most likely that this is not at fault, but it might be so I wanted to get a potential idea of what to look for and cost.

My problem is that under anything approaching heavy braking the ABS activates in such a way that the pedal gives and stays like that, i.e the ABS thinks one or more wheels are decelerating too fast and then even when the ABS blocks the brake line or pumps back the fluid then it stays like that. I have disabled the ABS and I can tell that the rears are braking too much with respect to the front.

Now, I think the problem is actually a fricition problem with one or both rear brakes not so much binding all the time but maybe seized caliper guide pins forcing one pad onto the wheel or a knackered caliper or something that causes one or both rears to brake way too much. BTW previous symptoms suggest I also had front brake problems but for sure this problem is now a rear brake problem. I should also point out I strongly suspect the ABS is not at fault.

The brake pressure regulator COULD be at fault on one or both circuits if it was, in some circumstances only maybe, failing to reduce the brake pressure to one or both rear brakes.

Hope it kind of makes sense,
Pete
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Old May 20th, 2006, 10:40   #7
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Pete,

This might sound a slightly unlikely cause but I experienced the ABS activating in, what I considered, "non abs" situations. At the time I had a dodgy front wheel bearing and when this was changed the problem went away. What I cannot say with any certainty is that changing the wheel bearing cured the problem because something else might have been disturbed/re-seated when the work was done............

Another thought - what tyres are you on? My car was always felt "nervous" on P6000's but I now run Contis and it's much better.
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Old May 21st, 2006, 21:29   #8
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Question Update: Non-hydraulic problem?

Latest news on this brake problem - see what you experts out there think because I think it is a friction problem of some description?

Over the weekend I took apart the right rear caliper and replaced the seals. Very light marks on the piston, rubbed it down with very fine abrasive paper, cleaned it all up and put back together. The right hand side was the one getting hottest.

Didn't get chance to do the left caliper because it has being raining most of the weekend and didn't fancy working in the cold & wet!

But on both sides I cleaned up the pads carriers and the pads, coated with copper slip and put it all back.

Now there were a couple of things of interest:
1) The discs need replacing. They are worn below the limit. I scraped off the rust lip round the top while I was there.
2) The outer pad on the left hand side (not the side which I removed the caliper) had more of a glazed appearance than the other three and a fine coating of what looked like rust. It was also not exactly the same thickness as the inner pad on that side.
3) The handbrake was clearly not binding, at least not when everything was cold.
4) The caliper guide pins were free to slide although since new rubber boots came with the repair kit then I overhauled the ones on the right rear while I was there.

One thing to note: the pads are new, they were changed just before I got the car < 2000 miles ago.

I put the fuse back in the ABS, cleared the fault code, adjusted the handbrake, put all the trim back and took it out for a drive.

It was certainly improved, the pedal was not hard and seemed as normal. After a while of gentle braking to make the rear pads bed back in following removal I tried some heavier braking. At first everything seemed ok but then it did the ABS problem again, but only a few times.

But then I worked it out - it seemed to be when everything was hot. As it had been raining and was cold the brakes seemed to cool down quickly. If I did a bit of heavy braking then I could get the ABS to come on but the pedal - and the ABS - behaved as you would expect (pulses through the pedal as ABS activates). But then short time later the pedal would feel hard again and then the pedal would 'give' with the ABS activating.

Diagnosis & Conclusions

To me the nature of the problem suggests a friction problem at one or both rear brakes. I no longer think it could be related to e.g the pressure regulating valve or maybe the master cylinder unless there was some strange fault that was intermittent but only occurred if the fluid got hotter and never affected the front. That would seem to odd to be possible. I think the ABS can be ruled out because it is just something that responds to the rear brake problem signified by a hard pedal.

I think that changing the rear discs is the logical next step because these are worn out. I think that the rear pads are likely not making proper contact with the disc for some reason not sitting were previous ones did and maybe contacting the disc rust instead, at least maybe this could explain the left rear outer pad appearance?

But also I am thinking at least one of the caliper pistons is sticking maybe so it is not retracting properly when hot causing the brake to bind and then under next braking that brake proceeds to start to lock? One thing I did was to rewind both calipers, obviously the one I had off but the other one as well so both have kind of been 'adjusted'. Also bled through the rear brakes fluid clean and no air (fluid was all changed recently).

Your opinions would be very much appreciated in this on-going saga. I don't really want to throw more money at the brake problem unless it is going to resolve it, I don't just want to change anything "just in case". There are also plenty of other things that need doing to this car.

The other thing is that I am, quite frankly, getting a bit fed up because this problem seems to be an "unknown" thing: if I know what is needed I can plan and budget for it. It is getting to the point where I am thinking of possibly getting rid of the car, if I cannot resolve this problem soon. I will not do that if I can spend just a little more to fix this issue because then I am back in "ok, that is a known problem just got to fix it then the problem goes away" territory and at least feel improvements are being made. Of course any other car - volvo or otherwise - would likely have other faults and I'd make a loss selling the car esp. so soon after purchase, plus if this car was fixed then I'd enjoy it a lot I am sure! But I am getting fed up and it might be better to "quit while I'm behind" as it were.

Pete
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Old May 21st, 2006, 21:39   #9
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Pete,

I don't think changing the rear discs will cost much as a dealer only charged me about 100 for rear discs & pads. Certainly as the discs are below the scrapping limit there *might* be a link. Brakes are something I'd not ignore under any circumstances.

Change them, and the pads, and hopefully your latest problem will go away.
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Old May 21st, 2006, 21:54   #10
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Yeah you're right, I can get mintex or ATE discs for 50 the pair, no labour as that won't take me long to do myself. Disc thickness was as low as 7.5mm according to my micrometer, the book says 8.4mm is the limit.

Pads are very new so I won't change them unless I really have to.

And yep, if someone said "pay that 50 for 2 new discs and it'll solve your problem" I'll do it like a shot.

But the thing is, what if it is new pads = 50, 2 new calipers = 80 each, 4 new pads = 30 and the problem is STILL there... do you see how that would hack me off, because I'd no doubt do it all in stages as well, i.e 2 discs - nope problem still there... change a caliper, nope still there.... etc.

The other thing that hacks me off is the discs must have been worn like this before I got the car. Did the RAC inspection pick it up? No. Did the b*****d garage who fitted the rear pads before I got it pick this up? No. I'm not saying they should have visually been able to tell but neither could I, but I thought "hmmm, those discs look near their limit might need changing soon..." then I checked and they needed changing beforehand. So why didn't anyone else think that? I'll tell you why, because the garage thought "hmmm... those discs might need changing soon, lets hope that mug buying this shed doesn't realise" and the RAC inspection man thought "hmmm... those discs need changing soon, I'd better ask for a larger bung in order to pretend I didn't notice".

Pete

BTW I am with the AA, although as it happens not because of the RAC-inspection-that-never-was.
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