brake doseur wanted

[ststech1973]

wanted good working brake doseur for xm s2 v6 24
mine has the usual problem of rear end sinking straight after start up then takes a while to raise often accomponied by loss of hydraulic pressure/brake pressure on lh display especially if you try to raise car manually it takes a while.
the hp pump is new all spheres have been replaced except the accumulator which i have ordered a new one.

[Dieselman]

Replace the accumiulator then see if the issue is resolved.

Your car only uses 2 pistons for the suspension system, so is only pumping 10cc per pump revolution. Without the reserve of pressure and fluid the Acc sphere provides, the system soon runs out of pressure when demand is high.

[White Exec]

Can confirm Will's point about the Accumulator reserve of pressure. Even with everything in good order, if suspension system pressure is allowed/made to fall very low, starting the engine and then putting the height lever to Intermediate or High without a pause will produce a Low Hydraulic Pressure dash warning. Message disappears, but car rises skowly.

[Dieselman]

Did you replace the anti-sink sphere? if not, the suspension will leak fluid when at rest, hence the falling when system pressure builds enough to operate the anti-sink valve.

[xantia_v6]

wanted good working brake doseur for xm s2 v6 24
mine has the usual problem of rear end sinking straight after start up then takes a while to raise often accomponied by loss of hydraulic pressure/brake pressure on lh display especially if you try to raise car manually it takes a while.
the hp pump is new all spheres have been replaced except the accumulator which i have ordered a new one.

Try the start-up procedure with the hydractive system disabled (pull a fuse). If the sinking does not occur, then you probably have a case of pressure equalisation as the hydractive valve opens. The later modified (Xantia) valves don't tend to do this because they equalise the pressure when you open the door, before anyone gets in to put weight in the car.

Did you replace the anti-sink sphere? if not, the suspension will leak fluid when at rest, hence the falling when system pressure builds enough to operate the anti-sink valve.

That is not correct information. The anti-sink sphere has no effect on the leakage or sinking of the suspension. The function of the anti-sink sphere is to provide a pressure reservoir for the rear brakes while the anti-sink valve is closed. On earlier sinker cars, the rear brake pressure reservoir was provided by the LHM in the rear struts, pressurised by the weight of the car.

[Dieselman]

Did you replace the anti-sink sphere? if not, the suspension will leak fluid when at rest, hence the falling when system pressure builds enough to operate the anti-sink valve.

That is not correct information. The anti-sink sphere has no effect on the leakage or sinking of the suspension. The function of the anti-sink sphere is to provide a pressure reservoir for the rear brakes while the anti-sink valve is closed. On earlier sinker cars, the rear brake pressure reservoir was provided by the LHM in the rear struts, pressurised by the weight of the car.

I don't agree.

The anti-sink valve creates a block to any hydraulic fluid leaking out of the suspension. The anti-sink sphere is used as the reserve pressure to ensure this valve stays closed until system pressure is attained by the pump. If the anti-sink sphere is low on pressure the valve opens and relases fluid from the suspension, thus the pump is required to refil the system.
There is no need for a reserve of rear brake pressure, as there is pressure until the rear is on the lower bump stops with the spheres fully exhaustedd of fluid.

If you have a hydraulic diagram depicting the operation as you describe, I would be keen to see it.

[djg]

We had the complete description in the guide, based on many available sources: http://citroen.tramontana.co.hu/en/susp ... ink-system" onclick="window.open(this.href);return false; It hasn't been contested since. So either the description is fairly accurate there or else...

[xantia_v6]

I don't agree.

The anti-sink valve creates a block to any hydraulic fluid leaking out of the suspension. The anti-sink sphere is used as the reserve pressure to ensure this valve stays closed until system pressure is attained by the pump.

The anti-sink valve is held closed by a spring, there is no need for any hydraulic pressure to close it. You may note that the front anti-sink valve does not require hydraulic pressure to remain in the closed position.

There is no need for a reserve of rear brake pressure, as there is pressure until the rear is on the lower bump stops with the spheres fully exhaustedd of fluid.

That statement is true for cars without anti-sink, the whole point of the anti-sink valve is that it disconnects the rear suspension cylinders from the (leaky) brake valve, to stop the suspension form sinking, thus when the valve is closed the suspension does not provide reserve pressure to the brakes, and the extra sphere was added to provide that safely function.

[xmexclusive]

Citroen booklet XM430-00/6 page 6 contains the statement:-
"The SC/MAC sphere returns fluid under pressure to feed the rear brakes when the rear SC/MAC valve is closed".

John

[djg]

It seems that *all* statements are essentially valid and any contradiction is probably just apparent. The misunderstanding might come from the expression of "feeding the rear brakes". The sphere does that under some circumstances but not when the car is in motion and the brakes are potentially operating.

When the car is in operation, the anti-sink valves are always open, so no description of what the sphere does with a closed valve will apply. When the car is stopped, the valve still stays open for some time. It only closes when the main pressure decreases enough to drop below the suspension side pressure. The anti-sink sphere simply provides additional pressure to make sure that the valve stays closed and doesn't reopen. It does so by adding an extra reservoir to the brake circuit side of the valve (because the brake valve, being usually the most leaky element, would deplete it). So, in this sense, it does feed the brake circuit, yes, but not in the sense that it would influence braking because the car had been stationary for long by then.

The sphere, by itself, when functioning normally, has no influence on the sinking, that's true. But if it doesn't hold pressure, the anti-sink valves will reopen and the car will sink rapidly. So, a bad sphere does have influence on sinking, and quite a drastic one at that.

Now, the question is, how is this different with the later Xantia valve? I could modify the illustrations in the description to account for that.