c5 brake pedal not always consistent

[jookul]

Hi guys

Im hoping u can help as my mechanic can not seem to find the problem.....and this may not be his fault. So here is the problem
Sometimes when I press my brake pedal it seems to be a bit harder than other times and I seem to loose a bit of brake pressure. That is I have to press the pedal a bit harder to stop the car.
Problem 2 again sometimes if I am stopped a say a set of lights and I have my foot on the pedal and the car is stopped I have noticed that without apply a ny extra foot pressure, the brake pedal will move down a tiny bit more for no apparent reason. I have also noticed that sometimes when you apply the brakes and the pedal is a bit harder and the car is coming to a halt you suddenly feel like the back brakes are biting and the rear of the car suddenly dips down
Problem 3 the brake pedal doesn't alway feel smooth when pressing it....it feels a bit gratey

I think this is all the same problem ..... iv had the car atvthe mecganic a number of times and he keeps telling me nothing is wrong with the brakes. We have replaced the master cylinder to no avail. The car has never failed to stop but I need this problem fixed. The car itsel is a c5 estate hdi 2001 with 163000 miles so it's not woth a lot of money. I did fibd one thread on the net of some guy who had the gratey pedal but there was no fix and I now xant find that again

Has anyone fixed this/these tupes of problems. ?...many thanks

[411514]

Master cylinder/vacuum pump/servo would be my first guess's. You say you have checked the master cylinder so it might be worth while checking the fuction of the other two. Possible ABS problems although I cannot really expand on that as I don't know a lot about the function of the ABS on the C5.

Probably worth giving the system a good bleed through if you havent already done so.
Might also help to dismantle the front brake set up, remove all the accumulated crap, clean & regrease caliper sliders and pad mountings. They are single piston calipers so sticky sliders could cause the 'grabby' feeling that you are experiencing.

Sam

[qprdude]

You dont give the year of your car, but there have been recalls due to brake assist problems on some C5's

http://www.dft.gov.uk/vosa/apps/recalls ... =Blank&tx=" onclick="window.open(this.href);return false;

Check number 10 specifically, although there could be more for different years.
You can tag your car year and model etc to your profile and set it to come up in your posts so everyone willo know what car you are asking advice about.