ZX rear brakes nightmare

Danbloke · Post by **Danbloke** » 31 May 2003, 14:20

My ZX 2.0i has just failed its MOT. The rear brake calipers had seized up and so I sorted them out, gave it some new discs and pads and took it back for another go.
Still no luck.
This time it's "low effort to the nearside rear service brake". The guy at the garage reckoned that the piston might still be seized, but I told him I'd sorted that out. After a bit of uming annd ahing he said, maybe the flexible hose is on its way out.
Does that seem likely?
The pedal doesn't feel spongy as I thought it would if that was going.
Is this guy mad? am I mad? What else could it be?
Any suggestions gratefully received as I've spent far too long underneath the car to give up on it now.

jeremy · Post by **jeremy** » 31 May 2003, 14:36

Is the back of the car lightly loaded and the fuel tank nearly empty?
If so the back brakes are not really meant to work. I know that this is a strange statement but there is a valve (or valves) somewhere which are designed to limit the rear brake effort when the car is lightly loaded to prevent rear wheel lock up. From what I remember of the Haynes book for our diesel there are a number of different mechanisms and so I would suggest you find which one is used in your car.
I have known MOT testers keep a load to be used in the back of some cars before tests to make sure this problem doesn't arise. I would suggest in any event that you have the car tested with some weight in it to prevent this problem.
jeremy

Danbloke · Post by **Danbloke** » 31 May 2003, 14:45

There is a rear brake pressure compensator valve for each side. But if that goes doesnt that mean that one side would be too high rather than the other too low?
That's the way I figured it would work, but of course I could be totally wrong [:D]

Dave Burns · Post by **Dave Burns** » 31 May 2003, 15:07

These valves can be pressure limiting, or in some cases deceleration cut off, which means a ball bearing inside the valve rolls forward under braking and operates a mechanism that prevents any more pressure reaching the rear brakes.
This type of valve has to be angled upward slightly towards the front to keep the ball bearing at its rearmost position.
This car will not benefit from added weight over the rear axle, the estate version will however, due to a load sensetive pressure limiting setup as it has massive (Iknow we're talking discs here) drums compared to the hatch.
Dave

Danbloke · Post by **Danbloke** » 31 May 2003, 15:54

I don't know how they work (Mr. Haynes doesn't bother to mention) but these are 4 or 5" x 1" hexagonal cylinders, mounted across the car, rather than along it (if you know what I mean)
does that help?

Dave Burns · Post by **Dave Burns** » 31 May 2003, 16:11

Yep pressure limiting, you could swap them over and if the problem then goes to the other wheel, money spent on a new one will be in the right place and will sort it.
My money would be on that valve but I would need confirmation of its condition before forking out.
If there is anything impeeding the progress of fluid along the pipework like a collapsing flexible hose, it may be reasonable to expect the same in the other direction, meaning the brake will not release instantly but will drag for a short while.
Dave

Danbloke · Post by **Danbloke** » 31 May 2003, 16:16

A way of testing it! I hadn't thought of that.
Thanks for that, I'll go and get my hands filthy and see what I can't find out.

Danbloke · Post by **Danbloke** » 04 Jun 2003, 02:44

It turned out it was the brake pressure regulator, and its now fixed and I'm back on the road again (yay!).
Thanks for the help, saved me hours of head scratching.

Dave1 · Post by **Dave1** » 02 May 2004, 15:43

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Dave Burns</i>

These valves can be pressure limiting, or in some cases deceleration cut off, which means a ball bearing inside the valve rolls forward under braking and operates a mechanism that prevents any more pressure reaching the rear brakes.
This type of valve has to be angled upward slightly towards the front to keep the ball bearing at its rearmost position.
This car will not benefit from added weight over the rear axle, the estate version will however, due to a load sensetive pressure limiting setup as it has massive (Iknow we're talking discs here) drums compared to the hatch.
Dave
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Dave,if you are out there, from the above, can I take it the pressure limiting valves will benefit from extra weight (eg, a tank of fuel) because my near side rear has just failed. Read about 100 on the test where as the other side was near 200. Worked well with handbrake. I have just put new pads in (after the test) as the outer one of the the nearside was very worn. The caliper was not silding very well so perhaps this was partly the issue. I was going to fill up with fuel before I go back on Tuesday anyway as I had read something on the site before. Will it help?
A quick response appreciated from anyone else who knows.
Dave

Dave Burns · Post by **Dave Burns** » 02 May 2004, 16:32

No it wont help at all, the rear disc brake set up is not load compensating.
Its purely hydraulic pressure limiting, the wheels will lock up at a known pressure at various speeds so the valves are there to make sure this pressure is never reached.
When they go wrong however they don't allow enough pressure through and so you end up with reduced brake effort on one of the rear wheels, or both if both valves go bad at the same time, which is rare.
Dave

NiSk · Post by **NiSk** » 02 May 2004, 21:32

From your discription. Dave 1, all your problems are associated with the non-sliding calliper - one working brake pad = 100, 2 working brake pads = 200? Too simple?
//NiSk

Dave1 · Post by **Dave1** » 03 May 2004, 01:31

Dave, thanks for your reply, I wondered how they worked and there is no point me going down that avenue.
NiSK, hoepfully you are right and the caliper will work better now it is sliding better. Both sides of the disc were getting used but the fact that the outer pad was worn substantially more than its eqivalent on the other side (to about 2-3mm) shows somthing was/is wrong.
Thanks all
Dave

Dave1 · Post by **Dave1** » 05 May 2004, 00:47

Passed today[:D]

aido · Post by **aido** » 22 May 2004, 03:45

my zx failed its 2003 mot due to low brake effort on rear o/s wheel,that was due to the rear brake compensator,it was really corroded and wasnt letting any braking pressure through at all.its the second one ive had fitted to the car in the 7 years ive owned it.