Hi Shaun,
Sorry for the delay in replying - I have seen your posts but every time I see them it's when I'm reading the forum on my phone away from home and by the time I'm at a computer where I have the data available and a proper keyboard to compose a reply on I've forgotten about it again.
The image you show is of an anti-sink valve, not the priority valve. They're quite similar in appearance, but perform very different functions.
A quick primer on the basics of the hydraulic system so you have an idea of what the purpose of each relevant component is.
High pressure is generated by the HP pump on the aux belt and is piped to the pressure regulator, which has the accumulator sphere attached to it as a buffer, and it has a simple pressure regulation system that consists of allowing the output from the pump to be fed to the sphere (and the rest of the hydraulic system) when the pressure is below 135 bars, and to be bypassed and sent back to the hydraulic tank when the pressure is above 170 bars. This way the pressure is kept regulated between those two figures when the engine is running despite variations in demand.
The first place this regulated pressure goes to is the priority valve. (Also known as security valve) It has 3 purposes but its primary purpose is to provide priority to the brakes over the suspension. It's a simple pressure operated valve - above about 100 bars all ports are open, meaning pressure is supplied to both suspension and front brakes. (Rear brakes are fed from rear suspension) Below about 100 bars the slide valve moves and the supply to the suspension is cut off, but the supply to the front brakes is maintained. This means if there is any fault causing a loss of pressure (such as a snapped aux belt) the supply to the suspension is cut off as "non essential" to allow the front brakes to work as long as possible from the pressure stored in the accumulator sphere.
The second purpose of the priority valve is that its a pressure distribution block - when open above 100 bars it works as a 4 way junction. One port is the input from the pressure regulator, one is the output to the front brakes (actually goes to the brake doseur valve) and the other two go to the front and rear anti-sink valves. The third purpose is it houses a pressure switch in it - this is the switch that causes the hydraulic light to come on when the pressure is low. The light being out with the ignition on means there is at least 100 bars of pressure available at the pressure regulator/accumulator.
In an older Citroen (before anti-sink) the pressure from the priority valve would then go straight to the height corrector which would supply the suspension when needed to top up it up, and alternatively connect the suspension to the overflow return to lower the suspension. The problem with this is that height correctors are inherently leaky in their design as they have no o-rings on the controlling shaft, (to make them as precise and sensitive as possible) this is the source of leakage that makes older Citroen's go down on their haunches between 30 minutes and a few hours.
Anti-sink was introduced on early Xantia's and consists of another slide valve which is inserted both before and after the height corrector, which when activated isolates the height corrector from both the suspension and the source of high pressure supply. This valve does have o-rings and has very low leakage rates. This dramatically reduces the overall leakage rate thus anti-sink Xantia's can stay up for days without losing much height. The valve is triggered to close off any time the pressure supply (from the accumulator sphere/pressure regulator) is lower than the pressure currently in the suspension. (It's a differential valve)
The priority valve is down near the firewall and has an electrical connector on it, whilst the front anti-sink valve is mounted somewhere under the hydraulic reservoir tank and looks similar but has no electrical connector. If its the one under the reservoir that's leaking then its the anti-sink valve. Apparently it's not uncommon for the o-ring in them to fail which can cause a massive leak, however it can be taken apart and have the o-ring replaced without too much difficulty. (Making sure its the right size and material for the o-ring) There are threads about this on the forum.
Finally here's a much better piping diagram of the suspension / anti-sink valves and so on. The piping is quite different for hydractive and no hydractive as well as the anti-sink valves themselves, and the document covers both so make sure you're looking at the right one.
(yours is non-hydractive) It should answer most if not all of your questions about what is piped to where...
http://homepages.igrin.co.nz/simon/imag ... /antisink/" onclick="window.open(this.href);return false;
, plus the issue with the lay strut is still there even after swapping the front spheres from one side to the other.
....
Out of interest, which pipes on the anti-sink valve were transposed ?