brake pedal drops

xantiadriver · Post by **xantiadriver** » 24 Oct 2009, 15:17

Helo all....I was wondering if anyone can tell me why my brake pedal sometimes drops and then raises again while still still slowing car down.Its been doing this for a while now. I have experianced pedal drop on cars with failing master cylinder/air in system but they never pumped themselves back up. When i start car in morning and wait for stop light to go off on dash there is no stopping ability until i I have pushed brake pedal along way ( i always do this before i leave the drivway). Brakes passed mot last week no problem...many thanks for any replies

1999 xantia 2.0 hdi lx

citronut · Post by **citronut** » 24 Oct 2009, 15:34

how fare dose the pedel drop as the way the doser valve works it cant drop by very fare at all,

the first thing to do is maybe fit a new acumalator sphere and bleed the brakes,

if there is air in the brake system this can give you the affect of no brakes on the first part of the pedel presure,

regards malcolm

xantiadriver · Post by **xantiadriver** » 24 Oct 2009, 17:16

When i first start car in morning and after waiting for stop light to go out on dashboard the pedal will go down way past halfway with no brakes at all...i take foot off then back on pedal and brakes work instant with barely any pedal drop and work fine through the day other than sometimes the pedal drops more than expected and pumps itself back up as if i was pumping brakes to expell air...but its the pedal pushing itself back up and not me. I have had to pump brake pedal on cars with air in brakes before but they never pumped themselves. I am familiar with brake systems but have no experiance of citroen...it seems to be an intermitant fault so are there valves in the system that makes it different to average cars?
thanks alan

citronut · Post by **citronut** » 24 Oct 2009, 18:28

dose the steering become heavy around the same time the brake's go,

with hydraulic citroens the working presure is there as soon as the system is presureised, and your foot only opens a vavle to varrying degreseto allow the presure through, normal braking or locking every thing up,

unlike other brake systems were your foot creates the presure as you press the pedel

regards malcolm

xantiadriver · Post by **xantiadriver** » 24 Oct 2009, 19:35

Thanks for reply...iv got problems with my pc hence my delay.
Ive not noticed steering because i test the brake before i get to the end of my drive which is short and straight line and the pedal drop if i am driving down the road is generaly while i brake in a straight line coming up to juntions/lights so i may go and test the brakes on a bend in the morning to find out. I should do some research to learn about the myserious citroen system

thanks... Alan

Post by **xantia_v6** » 25 Oct 2009, 07:55

Are you driving off before the STOP warning light goes out? The STOP light should only go off when there is enough pressure in the accumulator for the brakes to wok=rk normally.

This can take some time, but the power steering does not have a reservoir, so works immediately.

On my previous Xantia, it took 30 seconds or so for the stop light to go out on a cold morning, and if I was in a hurry I would use those 30 seconds to drive across the car park where I lived. Occasionally the pressure would not have risen by the time I got to the end of the car park where brakes were required, and I had to be quick with the handbrake, but as the brake pressure came up, you could feel the pedal rising.

A bit wiser now, I don't move a Xantia in the morning until I can feel (or hear?) the doseur valve working correctly, even if the STOP ligh has not quite gone out yet.

citronut · Post by **citronut** » 25 Oct 2009, 09:55

xantia_v6 wrote
"but the power steering does not have a reservoir, so works immediately. "

this may be so but if the drive belt was slipping or the pump was worn out, this can also affect the steering

regards malcolm

xantiadriver · Post by **xantiadriver** » 25 Oct 2009, 11:57

xantia_v6 wrote:Are you driving off before the STOP warning light goes out? .

No,i always wait for light to go out (10 seconds)before i move and test brakes long before i reach the pavement just in case. I cant test the car today but in the morning i may pump the brake pedal before i move the car.The brakes always work but im just a little concerned about the occasional pedal drop when im on thhe road. Is it possible its a tired hydraulic pump or accumulator or master cylinder

regards Alan

Old-Guy · Post by **Old-Guy** » 25 Oct 2009, 14:01

The braking system in a 'hydraulic' Citroen is quite unlike a conventional system. Instead of a master cylinder, there's a hydraulic valve (the 'doseur' valve) that controls the flow of high-pressure LHM from the main system into the brake lines.
I think the rate of fluid flow through the valve has to be quite low to prevent a correctly-working brake system responding too quickly for the driver.
BUT, if for any reason an abnormally high volume of fluid has to pass before all the pads start to press on the discs, the pedal will go to the floor for a frightening delay before the brakes bite.
Possible cause that I can think of:

- handbrake cable sticking (handbrake operates on front pads),
- damaged auto-adjuster on a front caliper,
- one or more pads 'knocked-back' (warped disc, sloppy wheel bearing, loose caliper),
- caliper sticking (extra travel when it frees-off),
- gas from a failing sphere in the brake lines,
- air in the brake lines.

The familiar air/gas problem doesn't square with an intermittent problem.

xantiadriver · Post by **xantiadriver** » 25 Oct 2009, 14:55

Thanks...i can rule out some of your suggestions because of it passing mot last week...i hope its just air in system but when i have had air in brakes on other cars it was perminant fault until i bled the brakes. i presume mot bloke would have spotted a failing sphere but i dont know

Old-Guy · Post by **Old-Guy** » 25 Oct 2009, 15:57

MoT tester should have spotted a failing suspension sphere, but unless he was a Xantia expert not either the rear 'anti-sink' sphere or the main accumulator.

I agree about air/gas.

I've just re-read your earlier posts and I think your problem is an almost flat accumulator or anti-sink sphere. My logic being that the pedal sometimes sinks because the suspension takes most of the available pressure in the system and then rises as the pump catches up with the demand. You can test each of them.

Rear (anti-sink) sphere: with the engine stopped and the tail-gate open, sit on the rear bumper. The car should sink and then rise back to normal height in about 20 seconds. Stand up, the car will rise then settle again after another wait. Depending on the model and how much fuel is in the tank, if the sphere is in top condition there may be enough reserve pressure to do this twice.

Accumulator (front): check the 'tick-rate' of the regulator valve - search the forum for a more detailed explanation.

xantiadriver · Post by **xantiadriver** » 25 Oct 2009, 16:48

Thankyou...i will test suspension activity in the morning
ragards Alan

1999 2.0 hdi lx