Xantia 2.1TD Brakes

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f00lzz
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Post by f00lzz »

OK will do... but that surely isn't right! When the STOP light goes out it should be ready to go..
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Post by Xaccers »

f00lzz wrote:OK will do... but that surely isn't right! When the STOP light goes out it should be ready to go..
When the stop light goes out it means it's up to pressure, if you do something to reduce that pressure such as raise the car to high or apply the brakes then the light will come on.
Normally the accumulators can compensate for the pressure loss when initially braking, but if they're both flat and the system pressure hasn't risen above the minimum needed to put out the STOP light, then pressure will drop the light will come on and then pressure will rebuild causing it to flicker until it rises above the minimum level again.
Once you've opened the doseur valve to brake it's pressurised it enough so you won't have the problem again until the pressure drops down.
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Post by KevMayer »

My understanding of the Xantia hydraulics, although not complete, is like this:-

The STOP light switch is on the Security Valve.

When the engine runs the hydraulic pump pressurizes the accumulator and the front brake system. When there's enough pressure, the security valve opens to allow pressure into the suspension and this is when the STOP light goes out.

So, if your STOP light goes out after a few seconds then it sounds like your accumulator sphere is flat because there's nothing to charge up, so, pressure build up is instantaneous. Another possibility I suppose could be a fault with the pressure regulator. That's if you're sure a new accumulator has been fitted.

When you press the brake pedal, there's a draw on the system and, because there's no reservoir of pressure in the accumulator, the system pressure drops and the STOP light comes on very briefly.

Another sympton of a flat accumulator I've seen in a few threads is of the brake pedal pulsing in time with the pump.

The brake system is a dead end system, so, there's no flow. Operating the brake pedal just opens the doseur valve to apply system pressure to the brake pistons.

When you lift the brake pedal the pressure comes off the brake pistons and, I think this is due to a bleed back from the doseur to the LHM reservoir. So, another possibility i suppose is a doseur valve fault where the pressure is bleeding straight back to the LHM reservoir. I'm not sure if this type of doseur fault is possible.

I'd put my 50 pence on a flat accumulator.
Cheers, Kev

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Post by Kowalski »

KevMayer wrote:My understanding of the Xantia hydraulics, although not complete, is like this:-

The STOP light switch is on the Security Valve.

When the engine runs the hydraulic pump pressurizes the accumulator and the front brake system. When there's enough pressure, the security valve opens to allow pressure into the suspension and this is when the STOP light goes out.

So, if your STOP light goes out after a few seconds then it sounds like your accumulator sphere is flat because there's nothing to charge up, so, pressure build up is instantaneous. Another possibility I suppose could be a fault with the pressure regulator. That's if you're sure a new accumulator has been fitted.

When you press the brake pedal, there's a draw on the system and, because there's no reservoir of pressure in the accumulator, the system pressure drops and the STOP light comes on very briefly.

Another sympton of a flat accumulator I've seen in a few threads is of the brake pedal pulsing in time with the pump.

The brake system is a dead end system, so, there's no flow. Operating the brake pedal just opens the doseur valve to apply system pressure to the brake pistons.

When you lift the brake pedal the pressure comes off the brake pistons and, I think this is due to a bleed back from the doseur to the LHM reservoir. So, another possibility i suppose is a doseur valve fault where the pressure is bleeding straight back to the LHM reservoir. I'm not sure if this type of doseur fault is possible.

I'd put my 50 pence on a flat accumulator.
My method of testing the accumulator sphere was to run the engine until the sphere was pressurised then turn it off. Next, get two people to sit on the lip of the boot, making the rear of the car drop with their weight. The suspension should try to correct for this extra weight, a good accumulator sphere should hold enough pressure to lift the car back up to the correct height. With my Xantias and a brand new accumulator sphere the car could do this twice!

Another possibility could be that the brakes have air in them and need bleeding, the first press of the brake pedal could be exhausting the pressure reservoir in the accumulator sphere compressing the gas in the brake lines.
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Post by f00lzz »

Hmm this is all excellent 'food for thought' but what about the fact that on level ground this morning it was OK... I'll get this mornings video up and see what you think!

So just to confirm.. this was this morning on my level drive doing what I consider to be 'normal'.

http://s69.photobucket.com/albums/i65/z ... 070091.mp4
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Post by citronut »

is the period of time similar to were it had been parked on a slope,

regards malcolm
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Post by RichardW »

Mine does this (or similar anyway - I don't get the STOP light coming back on, just no brakes) - and has done since I got it.... What seems to happen is that there is an internal leak somewhere which bleeds away the pressure from the main system, but the anti-sink valve closes and keeps the back suspension up. When the system repressurises and the anti-sink valve re-opens, the system pressure equalises, and the rear drops. In the interim period whilst the the rear suspension re-establishes itself the brakes don't work - probably becuase the low pressure at the rear diverts all the flow into the (non-working as there's no pressure) rear brakes. I suspect a combination of the flat rear anti-sink sphere and "air" in the rear brakes. It's worse when left for a few days (ie Monday morning if it's not been driven over the weekend), or if the last time it was used it had a heavy load in the back removed after the AS valve had shut (eg double trouble this morning, as I last used it on Friday afternoon when I collected a few 00 kg of firewood, which I then unloaded after a couple of hours stopped). No rear suspension or brakes for at least 300m this morning - fortunately it's near flat, and there's not too many folk about at 6.45 when I go out :roll:
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Post by f00lzz »

citronut wrote:is the period of time similar to were it had been parked on a slope,

regards malcolm
Yes Malcolm... overnight. Richards comment about a flat antisink sphere and the rear suspension taking all the pressure may be a clue because if it is on a slope, presumeably it is trying to 'self level'...

Anyway... popped down to my Sons house and started it this morning and all was OK! Something to keep an eye on though.. perhaps releasing the nut on the pressure regulator did the trick! A little trapped wind perhaps and don't we all get that sometimes!!!
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Post by citronut »

Richard wrote
"No rear suspension or brakes for at least 300m"

is that mm, meter's or mile's Richard :lol:

regards malcolm
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Post by RichardW »

Metres.....

Handbrake at the first T junction just past the drive, then have brakes by the time I get to the right turn up the road. Just got used to it I guess - my previous hydractive was similar....maybe one day I'll fix it. Then again!
Richard W
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