problem soft brake pedal

buckley · Unread post by **buckley** » 29 Jan 2009, 13:48

anyone help, I had the ABS light intermittant at first but now on all the time so the story begins
1/I had to replace the front pads, did this OK but also had to replace the front n/s caliper, bled system.
Next tried the brakes, ABS still on OK, but brakes seem to work OK, however when I park in the drive with the engine running the brake pedel goes to the floor when pushed, if I pump brake goes hard but again when pushed goes to the floor, I can't imagion its the master cylinder failing at the same time as changing the caliper, could the ABS fault be causing this?, no fluid level drop after doing the pumping either, Anyone any ideas? I am going to check sensors now to fix ABS maybe it will stop the problem

Dave

craigmanc · Unread post by **craigmanc** » 29 Jan 2009, 17:19

What vehicle is it? If it is a Synergie that is normal when holding brake pedal down.

AndersDK · Unread post by **AndersDK** » 29 Jan 2009, 18:04

Hi Dave -

Assuming its your "2001 c5 hdi lx done about 110,000 miles" we are talking about.

Your C5 have the conventional brakes system known from all other cars.

The ABS system has nothing to do with the brakes pressure. Its a completely self contained system that BLOCKS off your foot pressure on the brakes, to ease off a blocked wheel.
Thus a failing ABS wheel sensor setting the ABS fault lamp has nothing to do with your soft brake pedal.
The ABS system features its own dedicated computer.

Your most likely problem is still air in the brakes system.
It is VERY important to constantly monitor the brake fluid level, avoiding ANY possibility that the master cylinder is taking in air during your work bleeding the brakes.
All 4 wheels must be bled, including any bleed nipples on the ABS valve block.

The next most likely problem is a failing master cylinder piston seal allowing your foot pressure to leak back past the seal.

Fault tracing the ABS system is quite easy :
Each wheel sensor should have an ohmic resistance in the 1000 ohms region (exact value not important). Of course neither shotcircuited nor disrupted. Commonly they are disrupted when failing.
Unclip any connector found in/near the wheel arch and check each sensor.
As simple as that.
Its very unlikely to have other faults elsewhere in the ABS wiring.

Gregg1100 · Unread post by **Gregg1100** » 30 Jan 2009, 19:17

Hi,
To check brake hydraulics out on a non Citroen car (normal car,?)

Put brake hose clamp onto a front flexible brake hose--if pedal goes hard, fault is in that brake----air in system . Release clamp
Put clamp on other side--if pedal goes hard--etc. Remove clamp.
Some cars have a flexible brake hose at both sides at rear, some have only one- clamp them or it, and if pedal goes hard, fault in rears, air in system.
IF all flexible tubes are clamped, and pedal is still soft, master cylinder is at fault--seals.
If you don't have umpteen clamps, use molegrips with rag between jaws. They won't be on pipes long enough to harm.

Greg

citronut · Unread post by **citronut** » 30 Jan 2009, 21:36

sometimes when bleeding brakes with master cylender, you can kill old master cylender seals in the procces, as the piston's/seals will be travelling further than they do in normal use, and run through un polished cylender bore

regards malcolm

fred1 · Unread post by **fred1** » 31 Jan 2009, 13:31

Hi,

I understand that with the C5 it is necessary to disable the ABS system whilst bleeding the "primary Circuit " - which I take to be the master cylinder, lines to each wheel and callipers.

I also understand that connection to a Lexia system is needed to bleed the "secondary system" which I take to be the ABS and pressure assist feature for enhanced emergency braking.

From this I take it that it is likely that if you don't disable the ABS when bleeding the primary circuit you can get air in the secondary and will need to get access to a lexia to sort it out.

John

Gregg1100 · Unread post by **Gregg1100** » 31 Jan 2009, 19:09

Sorry---when I posted earlier, I forgot about crap abs systems differences.

Greg

buckley · Unread post by **buckley** » 31 Jan 2009, 20:26

Thanks for the help
OK I went to a garage and they checked the ABS with the computer (£30 mind), the rear left sensor was faulty, anyway last year I replaced the suspension arm bearings to do this I had to cut a re-solder the sensors on the back, the fault was a wire break just in front of the soldered joint, I have repaired and now the ABS works.

ON the brake pedal, I think I may be worrying over nothing, Its my sons car so I don't really now what it felt like before, any way I have fully tested all the wheels and they are all working, the car pulls up really fast and now the ABS is working I can feel it working under heavy braking. If you brake coming up to a junction or break heavy the pressure remains on the pedal and the pedal does not go to the floor. The only time I get the sinking is when I am stationary and I have pumped the brake up, if the engine is not running it does not loose pressure. A mechanic friend of mine says he often finds during MOT brake testing that alot a modern cars seem to have this type of creeping on the brake pedal and not to worry .
Dave