Just wondering

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Just wondering

Post by jvbram »

Hi, I have two Bx hatchbacks and both sink at the back overnight whereas the front takes longer. Is that normal? Cos if anything the front is heavier so i would have thought it would sink quicker. Jack

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Re: Just wondering

Post by Fumbler »

jvbram wrote:
Tue Nov 16, 2021 1:14 am
Hi, I have two Bx hatchbacks and both sink at the back overnight whereas the front takes longer. Is that normal? Cos if anything the front is heavier so i would have thought it would sink quicker. Jack
AFAIK it's a combination of the LHM molecules breaking down into smaller chains, making the fluid less viscous, and the fact that the rear suspension's design, as a trailing arm suspension and being at the end of the circuit, makes it easier to sink than the front.

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Re: Just wondering

Post by deltic »

Quite normal! Newer spheres tend to stay up longer, too.
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Re: Just wondering

Post by Kitch »

The back is further from the safety valve, and the return system is more open, so I just assumed that was the cause. I could be wrong.
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Re: Just wondering

Post by Kaapelimies »

The secret of the sinking rear is the brake valve that bleeds through a little. As rear suspension and brakes are connected. Oh, now it's not a secret anymore :/ :)
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Re: Just wondering

Post by white exec »

Just about all recent hydro' Citroens do this (except the 'anti-sink' SC/MAC models, which have an extra sphere and valves to prevent it). As Kaap says, there are more paths for fluid pressure leakage from the rear end, where brakes and rear suspension are linked.

When touring with our previous '89 1.9D, towing a caravan, we had to be careful when parking the hitched outfit for anything more than an hour or so, as the back of the car would sink, and hang its rear end from the towing ball/socket - the caravan having its corner steadies lowered. Our solution was to carry a short length of wood, which would be placed under the tow bracket, preventing the rear end sag.
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Re: Just wondering

Post by rutter123 »

Didn't early Xantia's (sinkers) suffer the same probs.
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Re: Just wondering

Post by white exec »

Yes they did - XMs too. It was why the SC/MAC system for these cars (aka "anti-sink") was introduced. It isolates the front and rear suspension units from the rest of the system when the engine is switched off. It is capable of keeping the car at normal height for weeks, under most conditions. SC/MAC consists of an extra sphere, two hydraulic valves and some extra pipework.
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Re: Just wondering

Post by jvbram »

On the japanese hydraulics diagram one of the returns, coloured red, goes to a Power Cylinder. Is that the security valve? Or is that named the priority valve? In which case what is the power cylinder?

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Re: Just wondering

Post by white exec »

Japanese hydraulics diagram?
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Re: Just wondering

Post by Jaba »

I presume you mean this: http://www.mars.dti.ne.jp/~ynar/bxorg_a ... 5/m15.html
I think that one of the power cylinders refers to the rear suspension cylinder and the one you mention possibly the pressure regulator. The security valve gives priority to the brakes when pressure drops hence its description as priority valve.
Its not a helpful diagram IMO as it is not 100% accurate and only shows the returns and is not labelled accurately due to the jp-en translation no doubt.
It might be useful when replacing the octopus though.
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Re: Just wondering

Post by jvbram »

Yes, that diagram. But the diagram uses the term Suspension Cylinders for the the front struts not Power Cylinder. It must be the Accumulator, no?