C3 (2004) - Brakes "sinking pedal" feeling - new master cylinder?

[mowgli]

So the brakes on this (new to me) C3 aren't functioning properly - you can feel the pressure when you first press the pedal, lets say you depress it to 50% and the brakes are engaging and it feels normal, but then after a second or so the pedal sinks further and you have to press quite hard to get them pressurised again.
I bled them with new fluid and the level wasn't low so I'm ruling out a fluid leak. I suppose there could still be air in the system at a place where my bleeding didn't clear, but after a quick google I've seen some suggestions that this could be an issue with the seals on the master cylinder.

Can anyone advise on whether this sounds likely, and if so, how easy is it to fit? I can pick up a second hand one for <£30 and am pretty competent with mechanics but I haven't worked on car brakes before. Are there any diagrams of the brake system knocking about?

Also worth mentioning that the MOT advisories showed "rear brake hose slight corrosion to ferrule" and "rear brake pipe corroded, covered in grease or other material".

Any advice much appreciated thanks

[Gibbo2286]

There's a couple of threads about this, try here fro a start.
viewtopic.php?p=771301#p771301

[PaulC5]

I would not bother with a used master cylinder, it may be no better or could be worse than the existing one. In the link above it mentions needing a diagnostic to bleed the brakes so if you do not have access to the equipment it will need to go to a dealer so it might be better to get them to fit the new part as well, that way you will get a guarantee. When checking the pedal do it with the engine off to see if the pedal still sinks since on some cars the pedal effort and how far it sinks is related to how quickly you press it with the engine on.

You could also do with checking the rear brake cylinders in case they are leaking which will mean removing the drums - note new hub nuts are needed each time they are removed as well as dust caps. This will also let you check the rear adjusters are ok and that somebody has not been over adjusting the handbrake cable which would stop the adjusters working.

[mowgli]

Yeah that's a fair point, I'll look into somewhere to get a new one. I've just read the bit about a primary and secondary system to bleed, the secondary needing diagbox - I did actually check and follow the service manual which basically described a normal bleeding procedure, so I'm not sure the 2-stage procedure described is relevant to my 2004 C3 (his is a 2012 C5). Hopefully thats the case as I don't really have the money to take it to a dealer.
Good point about checking it with engine on and off, I'll do that. And I'll look into removing the drums thanks!

[Armidillo]

Does it have ABS? As the linked thread explains, ABS & ESP are quite tricky to bleed properly, but it's obviously essential to get it right.

Another related issue - apparently ABS blocks can leak internally, giving symptoms similar to what you are describing.

[mowgli]

I don't think it has ABS, based on the fact that when I emergency brake, the wheels lock up :') Straight after an emergency brake test, the brakes are almost useless for about 10-30 seconds - i.e. I can press the pedal almost to the floor with very little resistance.

So, with the engine on, pressing the brake pedal is easy at first and I can hear air moving somewhere in front of the passenger side dash. It sounds like a hand pump/air moving between chambers in a suspension system (brake servo perhaps? although the noise doesn't sound like its coming from directly in front of the pedals which is where I asssumed the servo is). Once the pedal gets to about 50% it stiffens up a bit and the brakes engage. This is pretty consistent so when driving its at least predictable. Once stationary, the brake pedal slowly sinks further but the brakes are still engaged.

With the engine off, at first there is the same easy-press until 50% and moving air noise, but with each press the system increases pressure and eventually there is no air-moving noise and the pedal has resistance from the top.

[myglaren]

If it has ABS then there should be an indicator light up when the ignition is turned on, should then go out after a few seconds if the system self-tests as being functional.

[ozvtr]

I have found the C3's brakes to be hard and "snatchy". That is; it takes very little pressure application of the brake peddle to get the car to stop. And very little travel of the peddle to do so. It feels quite hard.
If the peddle feels "spongy" or there is no feel to it, then something is wrong.
If the fluid level in the brake reservoir is NOT going down then it might be an internal leak of one of the circuits of the master cylinder. It's not leaking fluid externally. AS you say there is no brake application for 50% of the travel that's another indication of one if the circuits failing.
It could also be the front pads and/or the discs worn beyond limits. They would need to be worn way beyond the limits but that's not beyond the realm of possibility.
The ABS module can be seen way down the bottom of the vehicle, under the near side headlight. The 6 brake lines can clearly be seen coming out of the hydraulic block. There is also a vinyl bag over the electrical connector. If it's not there, then you don't have ABS.
I have seen unscrupulous people blank out warning lights on instrument clusters so that the light never comes on (well it cant be seen).
IMHO you could roll the dice and try a used master cylinder. If it's cheap enough. I bought an air conditioning compressor for my son's utility for 20 pound! It runs perfectly!! Best deal I ever had! Having said that, we are talking about the braking system and it's safety critical!!! You don't want it to fail....EVER!!
It's your choice.

[ozvtr]

The service documentation doesn't mention requiring a LEXIA to bleed the ABS system. It is needed for the ESP system.
Air bubbles should be located in the upper part of the two pipes from the master cylinder. So just pumping the master cylinder should work them back up into the reservoir.
Looking on line, a brand new master cylinder doesn't look that expensive?