have to bleed my rear brakes every week!!

[runaraa89]

I have a Citroen Xantia 99 mod 16v 1,8i.
I have a brake delay on my rear brakes. When I bleed my brakes , air comes out and my brakes works fine. But after some driving my rear brakes have a delay again. I can't figure what it could be. I'm very tired bleeding my brakes all the time.

Do someone have a solution?! Many thanks.

[dnsey]

You may well have a dying suspension sphere, which is bleeding nitrogen into the hydraulic system.
What is the ride quality like?

[runaraa89]

then i think it is the middle suspension sphere. Because I only changed the suspension spheres on both side rear of the car. The ride quality is awesome, but the back end may sink a little after the car have been parked few days. sounds like a broken suspension sphere middle rear back?

[Mandrake]

Your anti-sink sphere will be internally punctured and bubbling nitrogen into the rear brake lines.

This is the sphere mounted in the middle at the rear with a pipe going directly into it.

We had exactly the same problem on my Dad's Xantia where it was needing the rear brakes bled every two weeks until that faulty sphere was replaced.

Be careful about how you remove it as it has a pipe union screwed into the sphere and the sphere itself is screwed into a threaded plate on the chassis.

You should remove the pipe union before unscrewing the sphere or you'll twist and snap the pipe. Remember to recover the oil seal for the pipe from within the hole in the sphere if your new sphere doesn't come with one.

[myglaren]

Have a look through this thread, lots of info there on changing the sphere, along with lots more useful if not critical information on the hydraulic system in eneral.

[Hell Razor5543]

When I fitted Gracie with new spheres I fitted one of the old O Rings (off one of the old normal spheres) between the anti-sink sphere and the bracket, to help assist in removing the sphere in the future.

[runaraa89]

Thank you very much!

I will try to replace the middle shere when i have time for it. looking forward to not need to bleed my brakes anymore.
what happend when the sphere is empty for nitrogen?

[Mandrake]

As the spheres age nitrogen gas very slowly diffuses through the membrane in the sphere into the oil side of the system then finds its way out into the atmosphere. As the gas volume shrinks the membrane starts to fold back tightly on itself until it cracks through fatigue. When this happens most of the nitrogen escapes quickly and the sphere is "flat" or punctured, however a small amount of nitrogen remains - this bubbles through the crack in the membrane into the oil at a much more rapid rate and finds its way into your brakes in the case of an anti-sink sphere.

The rapid build up of "air" in the rear brakes is a sure sign that there is a sphere with a ruptured diaphragm in the rear suspension somewhere. The anti-sink sphere is the only rear sphere that won't affect the ride quality if faulty.

[Old-Guy]

... The anti-sink sphere is the only rear sphere that won't affect the ride quality if faulty.

But it will mean no emergency braking (if pump pressure is lost) at the rear.

[Mandrake]

... The anti-sink sphere is the only rear sphere that won't affect the ride quality if faulty.

But it will mean no emergency braking (if pump pressure is lost) at the rear.

Correct, however I've sometimes wondered how effective rear brakes entirely on their own would be in stopping from any sort of speed - probably not that effective at all.

In cars that I've owned with the hand brake on the rear I could usually pull the hand brake on hard enough to lock the rear wheels and the car would hardly slow down at all - because all the weight throws onto the front wheels leaving no traction at the rear.

Consider also a push bike - its easy to lock the rear brake on its own without slowing down much but the front brake on its own will work so well that it will throw you over the handle bars... Both wheels and brakes are the same on a push bike, the only difference is the weight transfer.

I suspect on a Citroen the hand brake on the front wheels would be more effective for an emergency stop than rear foot brake only. (Not that I'm advocating not replacing the anti-sink sphere of course! )