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My friend has a 2005 Peugeot 206. He brought it by for me to advise him on the brakes, and when I test drove it the brakes were soft and spongy. He then took it to a mechanic, and when he picked the car up the guy told him the master cylinder needs replacing. Upon test driving the car, my friend found the brakes were now worse to the point of being almost useless, and there was now a hissing noise whenever the brake pedal was pressed.

So he brought it to me again and left it with me to change the brake fluid / bleed the brakes. I checked the reservoir which was full. I lightly pressed on the brake and it did not float down to the floor. Pumping the brake didn't improve the softness.

So I went ahead and bleed the brakes. Front wheel drive, so I did it in an X pattern ( FL, RR, FR, RL ). The fluid clearly hadn't been changed in a long time, it came out a brownish color and seemed to have allot of tiny specs of rust floating in it. There were allot of tiny bubbles, and one bleeder seemed to have a huge amount of air. I coated the bleeder threads and tube connection with silicon grease to make sure air wouldn't get in from the outside, the tube was curved up and the other end was submerged in fluid, so I'm fairly certain the air I saw came out of the brake lines.

Anyways, after finishing I took it for a test drive. At first the brakes still seemed a bit soft, but they firmed up after some use. I got going fast and was able to stop fairly quick, and the ABS seemed to be working correctly.

However, I still hear the hissing / pishing noise when I push down the pedal, and on gentle stops the brakes feel a bit weak, although not really mushy anymore. The front break pads where replaced about six months ago and look good.

So, what could the mechanic have done that would cause that pishing sound when the brake is depressed, and taking into consideration what I've done so far, what else needs to be done, tested?

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  • It depends on the car make, but the common way to bleed is starting with the farthermost wheel from the master cylinder, then progressively go to the next farthermost, etc. Example: RR, RL, FR, FL on left hand steering cars. A hissing when pushing, could it be air (or vacuum) escaping from the booster?
    – Aram Alvarez
    Jan 30, 2017 at 19:35

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Most likely brake booster has failed simple test , Start engine run for 5 minutes switch off let stand for 5 minutes apply brakes moderate pressure and hold restart motor if pedal sinks down about 1/2 " then booster is not holding vacuume and needs to be overhauled or replaced. MY experience 40 years working as a motor mechanic in the service station industry more recent 10 years as a brake repair specialist abd disc & drum machinist

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Does it make the same noise if you press the brake with the engine off? If not, it might be a vacuum leak from the servo or the vacuum pipe from the manifold.

Otherwise, check carefully for any signs of leakage from the lines - it's possible there is a pinhole leak somewhere. If you have an assistant, get them to press the pedal while you watch - but make sure you WEAR EYE PROTECTION while doing so - bake fluid in your eye would be a really bad thing!

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  • Pretty sure it did not. But what could the mechanic have done to cause this hiss?
    – Robert S. Barnes
    Jan 30, 2017 at 11:10
  • You'd have to remove the servo to replace the master cylinder - fairly easy to damage it or the hose in the process... Also possible that one of the connections to the new master cylinder wasn't done up tightly, but I assumed you'd have already checked that!
    – Nick C ♦
    Jan 30, 2017 at 11:13
  • That's the thing the mechanic did not replace anything as far as I know. The master cylinder looks like the original.
    – Robert S. Barnes
    Jan 30, 2017 at 12:01

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