Braking mystery…

slomofo · Feb 10, 2023

300tdi 90

Braking efficiency suddenly reduced in recent cold weather, they used to be mighty good but now they will not lock even on a wet road - so on a day off did some fault finding and have now exhausted my list;

Pads and Discs are low mileage and in A1 condition

Brake Fluid is 3Mths old

Vacuum Pump is working, though how much ‘suck’ it’s actually delivering isn’t exactly Reeperbahn quality

Non Return valve replaced and no change

No fluid leaks apparent

Pedal does not drop on start up.

So lack of ‘boost’ is my only conclusion, anything I’ve missed before I start buying parts?

lynall · Feb 10, 2023

Sounds like you may have a dead servo.

tottot · Feb 10, 2023

Test the vac unit. This is basic but does work, remove the pipe from the servo and with engine running put your thumb over the end of pipe. With the pipe hanging below your thumb it should stay on.

slomofo · Feb 10, 2023

tottot said:
Test the vac unit. This is basic but does work, remove the pipe from the servo and with engine running put your thumb over the end of pipe. With the pipe hanging below your thumb it should stay on.

I’ve tested the pipe from Vac pump to servo, it’s got suction but not knowing how strong it should be is the challenge - it has passed the ‘thumb test’ but it’s not what I’d call chrome off a trailer hitch strength, but should it be?

miktdish · Feb 10, 2023

Vacuum pumps will sometimes leak from the core plugs and the internal seal fails.
They are around £80 (unfortunately) but if yours is the original it's going to be around 30+ years old ..... so circa £2.50 a year ...
https://www.lrdirect.com/err3539-vacuum-pump-brake-300tdi

slomofo · Feb 10, 2023

miktdish said:
Vacuum pumps will sometimes leak from the core plugs and the internal seal fails.
They are around £80 (unfortunately) but if yours is the original it's going to be around 30+ years old ..... so circa £2.50 a year ...
https://www.lrdirect.com/err3539-vacuum-pump-brake-300tdi

It’s not an original pump from what I can see, it’s a black one (no silver metal face) but as to how old I know not (but if it’s a cheap one age is probably irrelevant) - as I don’t know how hard it should be sucking I’m not sure if it’s the source of the issue or the the Servo itself is FUBAR. I’ll swap out the whole assembly and cover all the bases. Then sell it!

Scooby22 · Feb 11, 2023

We can only speculate but it could well be the servo diaphragm leaking.

tottot · Feb 11, 2023

It passes the thumb test [ not a lot of suction at tick over ] and as there is no pedal drop when you have foot on the brake at start up most likely the servo.

slomofo · Feb 11, 2023

tottot said:
It passes the thumb test [ not a lot of suction at tick over ] and as there is no pedal drop when you have foot on the brake at start up most likely the servo.

I’m in denial - but it’s been whispering ‘Servo’ all the time….

Will be ordering bits then

Braking mystery…

slomofo

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lynall

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tottot

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slomofo

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miktdish

Guns n Chainsaws

slomofo

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Scooby22

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tottot

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slomofo

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