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300tdi brakes get progressively worse


dave88sw
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Hi,

I have a friend with a 300tdi auto, he says when he's driving it, the brakes are absolutely fine 90% of the time, however, when going down hill where he's forced to use them a fair bit the assistance gets less and less to the point he has to stand on the pedal.

I haven't actually seen the vehicle yet but he's asked me to sort it out for him. What do we think could be wrong with it? My thoughts are that the vacuum pump is getting weak and can't keep up but i guess it could also be a perforated servo? The pedal does sink when he starts it up etc and as i said, appears to function fine unless the brakes are used too much.

Anyone had experience of something similar? I don't want to buy a vacuum pump if i'm going to get stuck with it.

Thanks

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I appreciate the suggestion but from what he describes the problem goes a little deeper than brake fade, he says it's virtually impossible to stop it and has parked it up until it's fixed. He also said it never used to be like it.

I guess i need to go and see it and work out what it is but he's 30 miles away so was hoping to save a trip.

Cheers

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Sounds like either a vacuum pump failure or a split in the servo (where the master cylinder bolts on) to me. I've had both issues.

My monies on the vac pump though, otherwise i'd expect the loss to be pretty much constant. Mine had clearly been failing for a long time until one day it gave in completely, quite frightening.

Changing it transformed my opinion of Defender brakes.

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It's also is a symptom of brake fade, something I've never had although have the same brakes as a tdi although I don't have a vacuum pump to go wrong but the lack vacuum would make the pedal harder to press would it not.

As there's no info on the condition of the brakes I suspect your friend's disco has brake fluid hasn't been changed in a while and may also have iffy pads both items are a course of fading, or to change his driving style may help and select a lower gear ratio when descending a hill.

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Vacuum pump...... going downhill in an auto he'll almost certainly not be using much if any throttle therefore with a weak pump I suspect he's using more vacuum than is being created.....

Worth checking the hoses first!!

This is what's happening :)

The old Ford Kevscort vans did this from the factory if you went downhill on idle....

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Vacuum pump...... going downhill in an auto he'll almost certainly not be using much if any throttle therefore with a weak pump I suspect he's using more vacuum than is being created.....

Worth checking the hoses first!!

This was my thought too so i'll be looking here first.

I definitely take on board the comments about brake fade and will have a thorough check of the condition of discs and pads when i see the car but from what he says to me this developed quite quickly. He's had the car a number of years so unless the pads and discs are getting very thin i don't think it's him riding the brakes that are causing the problem.

Thanks guys

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A quick test - with the vehicle parked and engine idling, press the brake pedal. If after a few seconds you start to feel the pedal pushing back harder & harder under your food (feeling more solid), it's a brake vacuum fault.

Hose leakage is most likely - check the condition of the hoses and unions. Then the diaphragm, then the pump.

Si

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Thought i'd update this with the outcome. Went over to see my mate, pulled off the vacuum hose and fired it up and it had really strong suction. Decided to take it for a short drive to see for myself what the brakes were like. As he said, after a few minutes there's very little assistance so i pulled over and pulled the pipe off again. This time there was little to no suction on the pipe. So, as a few suggested and as i initially thought, it would seem the vacuum pump is getting weak. It's also leaking oil so that's a pretty sure sign that it's not in the best of health.

Thanks very much for the help guys

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