another question on brakes

Mark Disco · Oct 4, 2009

before i buy a new master cylinder i wanna check its the right fix for my problem ....

300tdi auto and when i press the brake pedal it takes an age to stop, feels like the discs are covered in a light smearing of grease. turns out that im not getting full braking at the rear, neither of the rear discs are getting very hot on a drive and theres a layer of rust round the edges of the discs that isnt going away indicating that the rear pads arent putting enuff pressure on the discs. i pulled the pads out in the week (the pads are only about 2k miles old) and it looks like the caliper pistons are working ok, they are relatively easy to push in using a g-clamp and they come out when the brake pedal is pressed, and there's no scoring on the piston shaft. would a faulty master cylinder cause only the rear brakes not to work? or is there something else that could be at fault? both front brakes look to be working ok, no lips on the discs and the pads are brand new. there was a rusty lip on the rear discs that ive cleaned off and the discs are now pretty flat but the pads havent been making contact with any of this area and there are a few rusty marks on the middle of the pad contact area that have been there over a week (the kind that a pad would normally remove).

any help much appreciated, i dont wanna spend £124 on a new master cylinder if it isnt the fault.

discool · Oct 4, 2009

What you say seems ok to me, the rears do have that lip (5 or 10mm) Perhaps your pads have not bedded in yet, are they of a known manufacture as there's some iffy counterfit ones on the market. Do you have abs ? when did you last change the brake fluid ? changing the fluid can make a lot of difference.

Mark Disco · Oct 4, 2009

the pads are Mintex, both fronts and rears were changed along with the brake fluid about 2000 miles ago. the lip on the rear discs is about 10-15mm the pad should cover at least half of where the lip is but nothing is happening. when you look down at the pads when somebody presses the brake pedal you can see the pads move slightly but when driving there doesnt seem to be much braking. dont think its pads bedding in as the brakes were working ok untill about 300 miles ago and they seem to be steadily getting worse. car is fitted with ABS but the pump is disconnected due to a fault with 1 of the sensors, could that be stopping force to the rear brakes?

nicedayforit · Oct 4, 2009

Mark Disco said:
the pads are Mintex, both fronts and rears were changed along with the brake fluid about 2000 miles ago. the lip on the rear discs is about 10-15mm the pad should cover at least half of where the lip is but nothing is happening. when you look down at the pads when somebody presses the brake pedal you can see the pads move slightly but when driving there doesnt seem to be much braking. dont think its pads bedding in as the brakes were working ok untill about 300 miles ago and they seem to be steadily getting worse. car is fitted with ABS but the pump is disconnected due to a fault with 1 of the sensors, could that be stopping force to the rear brakes?

Is the brake servo operating correctly? Worth checking the non-return valve for leakage. Assuming it's a diesel, also check the vacuum pipe connections at both the servo and the vacuum pump, the rubber pipe connections tend to split and leak.
Do you loose brake fluid at all?
Your vacuum pump could be on its way out (if you have one).
If your not getting full braking at the rear but ok at the front it's possible the pressure reducing valve is faulty.
Your brakes should work ok irrespective of the abs pump being disconnected.
After all that it could be the master cylinder.
How old is your car?

Mark Disco · Oct 4, 2009

the vac pipe from the vac pump to the servo is ok, it look like recent replacement and there are no splits in it, the vac pump also looks like a replacement. the non-return valve is the plastic thing just infront of the servo with the pipe attached to it? il go and check that to make sure its ok. no checked the servo itself but it seems to operate as normal, press the pedal repeatedly and it gets harder, start the car and the pedal sinks slightly.
there is no loss of fluid at all, and no ingress of air into the system, ive bled it maybe 3 or 4 times and the last 2 times theres not been a single bubble come out.
pressure reducing valve? kinda like a bias valve system? does the disco have 1 and are they problematic? and is there a way to test it if i do have 1?
car is an N reg, so 1996 i think?

nicedayforit · Oct 4, 2009

Mark Disco said:
the vac pipe from the vac pump to the servo is ok, it look like recent replacement and there are no splits in it, the vac pump also looks like a replacement. the non-return valve is the plastic thing just infront of the servo with the pipe attached to it? il go and check that to make sure its ok. no checked the servo itself but it seems to operate as normal, press the pedal repeatedly and it gets harder, start the car and the pedal sinks slightly.
there is no loss of fluid at all, and no ingress of air into the system, ive bled it maybe 3 or 4 times and the last 2 times theres not been a single bubble come out.
pressure reducing valve? kinda like a bias valve system? does the disco have 1 and are they problematic? and is there a way to test it if i do have 1?
car is an N reg, so 1996 i think?

From what you say it sounds as though .your servo is ok.
Yes these discos have a prv (bias valve). Follow pipes from abs pump, it's just below. They can apparently cause problems although I've not had that experience. No way to test it that I know of.
Do you know if the brake discs have ever been replaced, if not they are overdue whatever they look like. Cheap to replace.
Just a thought, you've not got any free-play in the rear wheel bearings by any chance?

thebiglad · Oct 4, 2009

Mark Disco said:
before i buy a new master cylinder i wanna check its the right fix for my problem ....

300tdi auto and when i press the brake pedal it takes an age to stop, feels like the discs are covered in a light smearing of grease. turns out that im not getting full braking at the rear, neither of the rear discs are getting very hot on a drive and theres a layer of rust round the edges of the discs that isnt going away indicating that the rear pads arent putting enuff pressure on the discs. i pulled the pads out in the week (the pads are only about 2k miles old) and it looks like the caliper pistons are working ok, they are relatively easy to push in using a g-clamp and they come out when the brake pedal is pressed, and there's no scoring on the piston shaft. would a faulty master cylinder cause only the rear brakes not to work? or is there something else that could be at fault? both front brakes look to be working ok, no lips on the discs and the pads are brand new. there was a rusty lip on the rear discs that ive cleaned off and the discs are now pretty flat but the pads havent been making contact with any of this area and there are a few rusty marks on the middle of the pad contact area that have been there over a week (the kind that a pad would normally remove).

any help much appreciated, i dont wanna spend £124 on a new master cylinder if it isnt the fault.

When you put the new pads in, what procedure did you adopt? The normal, light braking for the first 100mls or so?

That's what I used to do until I read some stuff on t'internet about bedding in new pads. I tried this method and it worked very well for me, as I had trouble on a Contrôle Technique (mot) with the back brakes giving a poor reading on the test machine.

This is the method, it's up to you whether you believe it or not, but it has worked for me on several vehicles:

1. Find a quiet bit of unrestricted road, get up to 60mph. When at constant speed and assuming it's safe to do so, brake as hard as you can down to 30mph. Then go back to 60 and do it again.

2. Do this 8-10 times and then you are done and your brake pads are bedded in. Don't worry if you smell some rubber, your tyres will be given a work-out too.

After doing this on the car that had bad back brakes, I took it back to the test centre and she passed with flying colours.

Mark Disco · Oct 4, 2009

nicedayforit said:
From what you say it sounds as though .your servo is ok.
Yes these discos have a prv (bias valve). Follow pipes from abs pump, it's just below. They can apparently cause problems although I've not had that experience. No way to test it that I know of.
Do you know if the brake discs have ever been replaced, if not they are overdue whatever they look like. Cheap to replace.
Just a thought, you've not got any free-play in the rear wheel bearings by any chance?

there's still plenty of meat left on the discs, nowhere near needing replacing but ive got a full set of 4 to go on anyway. didnt replace them when i did the pads cause i didnt have a hub nut socket at the time. all the wheel bearings are ok, no play at all in any of them and no strange rumblings or noises. might see if i can pick up a used bias valve just in case its that, if its cheap its worth a try!!

ive never driven carefully for 100 miles EVER

when i fit pads i de-grease them, give them a quick rub on some 120 grit paper, then go for a drive building pressure on the brakes gradually. after 5 miles or so i find a nice quiet road and do a few emergency stops just to make sure the pads are well bedded in. all the brakes were working perfectly when the pads were 1st fitted about 5 months ago, its only been the last 3 weeks or so that ive noticed them getting worse. they've settled down in the last week or so to just being plain crap!!

spendrey · Oct 5, 2009

Hi,

I had this exact problem on my Disco until a few weeks back... we replaced both the Servo and M/Cylinder (because they came together from a local breaker), but I'm 95% sure it was the Servo. I did the 'tests', and thought I could feel the drop after building up pressure then switching on the engine, but once replaced it was nothing like what really should happen. A good drop of 2 inches or so is what I get now, and of course, good strong brakes!

We also had a lot of crap in the brake fluid which was bled through, but I don't think this was the cause because of the servo test symptoms I just mentioned.

I'd recommend a servo replacement if you're going for 1 component at a time... and good luck!

Scott

andyb66 · Oct 5, 2009

Mark Disco said:
when i fit pads i de-grease them, give them a quick rub on some 120 grit paper,

Can I also suggest that when you fit new pads to old discs you gentle file off the edge of the pad so that none of the pad is running or acting upon the rusty outer edge of the disc face. This doesn't appear to be your problem but worth bearing in mind.

I would have the pads out and check for glazing in case. And check the disc thickness too. IIRC they are about 13mm thick from new. I have just advised the replacement of a pair of fronts that are down to just under 9mm thick.

Have you checked the flexible hose on the top of the rear axle? Perhaps it is collapesed internally and not delivering enough fluid??

another question on brakes

Mark Disco

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discool

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Mark Disco

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nicedayforit

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Mark Disco

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nicedayforit

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thebiglad

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Mark Disco

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spendrey

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andyb66

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