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Is this a servo or vacuum pump fault


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Hi there, yet again the brakes on my 300tdi defender 1997 have come to my attention as they are poor. The reason its come to mind is i had to do an emergency stop the other day and only just missed the other driver as it takes so long to stop. When i do an emergency stop i cant lock the wheels no matter how hard i try, and the force i am applying onto the peddle is alot, im nearly of the seat! They are just not up to scratch when braking hard.

I have checked the brake vacuum pump for suction where it attatches to the servo and its sucking a fair amount to feel it on my thumb. Whern the engine is started, the pedal does sink, so does this mean that the servo is working? As soon as i turn the engine off i loose total servo assistance instantly... Whereas i thought the vacuum is maintained for atleast 10 mins?(correct me if im wrong). Another thing i have noticed, is a hissing noise when the brake pedal is depressed and when the engine is turned off after it being running you can hear air escaping from somewhere when the pedal is depressed. I have checked for cracks in the servo and i cant seem to see any and ive also checked the seal between the master cylinder and servo and that seems fine.

6 months ago the defender had a new master cylinder but there are bubbles coming up out of the reservoir when the pedal is depressed (is this normal then). Its also had new brake pipes all round including flexis, new brake pads and i rebuilt the rear brake calipers. The system has been bled that many times that it cant be any cleaner;) .... (but that was a previous brake problem that is now sorted)

Any help would be greatly appreciated folks, thanks in advance

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Hi there, yet again the brakes on my 300tdi defender 1997 have come to my attention as they are poor. The reason its come to mind is i had to do an emergency stop the other day and only just missed the other driver as it takes so long to stop. When i do an emergency stop i cant lock the wheels no matter how hard i try, and the force i am applying onto the peddle is alot, im nearly of the seat! They are just not up to scratch when braking hard.

I have checked the brake vacuum pump for suction where it attatches to the servo and its sucking a fair amount to feel it on my thumb. Whern the engine is started, the pedal does sink, so does this mean that the servo is working? As soon as i turn the engine off i loose total servo assistance instantly... Whereas i thought the vacuum is maintained for atleast 10 mins?(correct me if im wrong). Another thing i have noticed, is a hissing noise when the brake pedal is depressed and when the engine is turned off after it being running you can hear air escaping from somewhere when the pedal is depressed. I have checked for cracks in the servo and i cant seem to see any and ive also checked the seal between the master cylinder and servo and that seems fine.

6 months ago the defender had a new master cylinder but there are bubbles coming up out of the reservoir when the pedal is depressed (is this normal then). Its also had new brake pipes all round including flexis, new brake pads and i rebuilt the rear brake calipers. The system has been bled that many times that it cant be any cleaner;) .... (but that was a previous brake problem that is now sorted)

Any help would be greatly appreciated folks, thanks in advance

From what you are saying, and ignoring the bubbles in the master cyl issue, it does sound like you have a defunct servo - maybe a small hole in the diaphragm, which doesn't completely stop it working. The other thing might be the non-return valve leaking where the vacuum hose enters the servo body - or if that were leaking it could allow air to leak back in when the engine is stopped. If the hissing happens when the engine is running, and it's not a leak into the pipe (eg where the egr valve once connected)then it points to a leak inside the servo. I can't see that with the engine and therefore vacuum pump running, as it was the other day even if there was a small leak in pipe, nrv or pump that you would get no assistance. So I'd say you need to try another servo. And they are easy to change.

Nigel

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Thanks for the reply chaps. Looks like i should try a new servo then. As i have checked the non-return valve and can only blow into it the one way. I dont think the pipe is leaking, as i have felt along it and i cant feel any suction even when my finger is over the end. The hissing when the engine is running can be heard in the engine bay as well as the footwell.

Josh

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It seems to me that you have 2-faults. The hissing is loss of vaccuum (open the bonnet while the engine is running, then pull the fuel solenoid wire off so the engine cuts out). You may well be able to better tell where the hissing is coming from.

Bubbles rising in the reservoir is usually failure of the master cylinder seals.

Les.

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Thanks for the reply Les. Surely the master cylinder cant give up this early. Its been on 6 months or less and about 500 miles. Its genuine LUCAS! To be honest, i cant tell where the hissing is coming from, as ive pritty much done what you said. Although, it can be heard in the footwell...Is it time for a new servo? or am i better getting it checked out by someone, as ive pritty much replaced the whole bloody brake system

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