Typsey Posted August 23, 2009 Share Posted August 23, 2009 Hi guys, 95 M Reg 300Tdi. I changed the front pads a couple of weeks ago and their has been a hissing noise coming from behing the front dash ever since. It disappears once you depress the brake pedal but comes back soon after. Once I cut the engine, you can hear it hissing as if pressure is released. Any ideas? No fluid loss or braking performance loss. I look forward to hearing your suggestions. Typsey. Quote Link to comment Share on other sites More sharing options...
WESTENDER Posted August 23, 2009 Share Posted August 23, 2009 vacuum leak on the servo? Quote Link to comment Share on other sites More sharing options...
Typsey Posted August 23, 2009 Author Share Posted August 23, 2009 vacuum leak on the servo? How would I be able to tell? To be honest it is what I have been thinking but what could have caused it? And is it an easy fix? Thanks for the reply Westender. Quote Link to comment Share on other sites More sharing options...
errol209 Posted August 24, 2009 Share Posted August 24, 2009 How would I be able to tell? To be honest it is what I have been thinking but what could have caused it? And is it an easy fix? Thanks for the reply Westender. Yep, sounds (geddit) like the servo. It could just be age. Why it isn't affecting brake performance I don't know. Some ways to tell what's going on: Pop the bonnet and look for an inch diameter pipe coming out of the vacuum pump on the engine and going to a plastic connector (non-return valve) on the brake servo. Whip it off and see if it's split anywhere. Then get the non-return valve off (you should get a huge inrush of air) and try blowing and sucking through it. If it passes air both ways it's duff. While the pipe is off, start the engine and stick your thumb over the end. Noise still there? then its the pipe. The vacuum line is used for other things like the aircon, if you have it, so you'll need to check all that pipework. The fact that it goes away on braking suggests the servo air valve is letting air through all the time, hard to eliminate, but there is a filter and valve overhaul kit for most servos. Assuming you didn't have the master cylinder off (no need for a pad change) and everything else is OK, then the only other thing it could be is a small hole in the diaphragm inside the servo unit. This is user-serviceable if you can find the parts (the overhaul kit doesn't include the diaphragm) but not physically easy to do. Quote Link to comment Share on other sites More sharing options...
Typsey Posted August 24, 2009 Author Share Posted August 24, 2009 Very many thanks for that. Very comprehensive and clear. Is this an MOT failure? Quote Link to comment Share on other sites More sharing options...
errol209 Posted August 24, 2009 Share Posted August 24, 2009 Is this an MOT failure? Not if the brakes still work to spec. Quote Link to comment Share on other sites More sharing options...
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