Sinking feeling

[duncr]

Hi ... new member with a 2006 W203 C180K Estate. Car's been in the family since new and I've been driving it for the last 6 years. Have been having a brake problem, as follows...

Brake pedal was sinking slowly to the floor, accompanied by comensurate loss in braking efficiency. Braking system otherwise in good shape; all calipers, hoses, pads & discs around 2k miles old, no leaks, fluid level normal and stable (i.e. no fluid being lost). Given all of that, I suspected the master cylinder.

I replaced it this afternoon with a new part. Bench bled the cylinder before installing, then did a full fluid change / pressure bleed on the whole system. Unfortunately, the pedal still goes slowly to the floor. With engine off it firms up nicely after a few pumps, and holds as it should. As soon as the engine is started and vacumn builds up it starts sinking.

Can anything else cause this problem, or have I been unlucky and got a faulty new master cyclinder?? Any advice gratefully received...

 

[Petrol Pete]

Hello and welcome . Have you tested the vacuum side of the system ?

 

[duncr]

Thanks for the reply. I've checked that there is vacuum, and I removed the vacuum tube and blew from each end to test the check valve, which seems to be ok. Wouldn't lack of vacuum cause the opposite effect though, i.e. a hard pedal?

Where the vacuum hose meets the booster there is another small-diameter vacuum pipe coming off, which disappears under the scuttle - I haven't been able to trace the end of it. Any idea what this is, or if it's relevant?

 

[markjay]

On other thing could be the flexible brake hoses, if one if them has failed then it could expand under pressure. But if this was the case then you wouldn't have it working properly immediately after starting the car, which you say is the case.

 

[markjay]

I think you're trying to suggest that there's an 'over vacuum' in the system, and that this is what's causing the brake pedal to sink, but I have never encountered such a fault, instead in my experience with sinking brake pedals is that it due to either a faulty or 'reversed' seal in the master cylinder (or bore pitting due to water ingress and corrosion), or air bubbles in the system, or pressure loss resulting in fluid escaping e.g. at the master cylinder, brake hoses, or calipers.

But you are saying that there's no brake fluid loss and that the brake fluid level remains the same?

Also, with ABS, there may be additional measures that need to be done (using STAR?) to bleed the ABS unit, I don't know.

 

[duncr]

Thanks for all the suggestions. The hoses are all less than a year old and I've had the car on a friend's lift today and thoroughly checked all the lines again - no sign of leaks, and as I mentioned there has been no drop in fluid level in the reservoir. I belive the vacuum system is operating normally, all the tests I've found to carry out suggest that it is. I changed the master cylinder yesterday, so I hopefully the brand new one is not faulty but obviously that's a possibility. The whole system was pressure bled at about 28psi, and the master cylinder bench bled before installing. I'm not aware of any special technique to bleed the abs on W203s, but again that's a possible issue.

My plan of action next weekend is to blank off the master cylinder ports and see if the pedal still drops. If it does then it must be a faulty new master cylinder, but if it proves to be ok the only thing left must be air so I'll try bleeding again. I've read that bleeding with the engine on helps with W203s - presumeably something to do with the ABS pump. Any more tips gbratefully received!

 

[heybuddy]

duncr, did you ever get this figured out? I have a very similar problem on my 83 W126.

Thank you

 

[duncr]

Yes, I got it sorted in the end. I proved the master cylinder was OK by blanking off the ports, which gave me a rock-solid pedal under all circumstances. I reconnected the lines then pressure bled again, which improved things slightly. I then re-connected the pressure bleeder and cracked each the lines on the ABS module open in turn... messy, but got a little air out of a couple of them. I then went round all all four corners again, and finally got a good solid pedal. With hindsight, I think bleeding the old-fashioned way with good, firm strokes on the brake pedal would probably have been more effective at driving out the air that it seems was trapped at the ABS module. Good luck with yours.

 

[AndrewOl]

A vacuum bleeder sorted a similar problem on my A207.