Priority Valve - Stripdown And Overhaul

As I prepare to reinstall the hydraulics, I serviced the Priority Valve.

A DS priority Valve

The priority valve was fitted to DS models from December 1967 onwards. So my car would have been among the first to have them fitted. It was introduced as a safety feature and replaced a distribution block that had some of the same function. The function of the priority valve is, in the event of major loss of hydraulic fluid, to divert fluid and pressure from the suspension circuits to the brake circuit. The brakes are given priority. Hopefully for reasons that are obvious to you. Generally, ID cars have a different brake mechanism and so, instead of a priority valve, they have a ‘security valve', but the two have a lot in common. I think that the DS brake system has an additional safety valve and a switch for a 'low pressure' red warning light on the pedal assembly. The simpler ID system doesn't have that extra safety valve and it's equivalent to the priority valve has a red warning light switch fitted.

The priority valve is essentially a non-return valve. Under normal conditions (normal hydraulic pressure) a spool valve - compressed against a spring - allows fluid to flow through all appropriate circuits. When there is a significant fluid loss and drop of pressure, the spring is able to expand and push back against the spool valve - effectively closing one fluid route and diverting fluid and pressure the the brake circuit. You will understand the benefit of being confident that the priority valve is working properly.

When you start the car, pressure starts to build. Pressure is fed initially to the brake circuit. 

Initial pressure to the brake circuit

As pressure builds it is sufficient to compress the valve against the spring and allows access for  pressure also to be  fed to the suspension circuit.

Spring compresses: pressure now fed to the suspension circuit

So if you are a DS driver, now you know why, when you start the car, the red low pressure warning light goes off, and then the suspension rises. As you can probably work out, if there is a sudden drop in pressure, the spring is able to expand shutting off the suspension circuit - but not the brake circuit.

Sometimes the spool valve can stick. So if you have problems with your suspension, there is an outside chance its the priority valve playing up - though height correctors would be the prime suspect.  If it is sticking, its because muck has gotten into the hydraulic circuit. It's also not unheard of for the push-fit overflow return pipe to fall off the left hand end - causing a leak of hydraulic fluid under the car. In that case you can simply push the pipe back on. But you won't know that's the problem straight away as the priority valve is hidden away.

The priority valve is tucked away behind the metal cover and against the firewall in the left hand front wheel arch. You'll need to remove the wing to remove the cover as one of its bolts is inaccessible otherwise.

Priority valve in the wheel arch
(photo credit Hugh Parris)

This was the state of mine. Can you see it?

My priority valve

You'll need to remove some of the hydraulic pipes to gain access to the valve, as well as four hydraulic pipes and a return line on the valve itself. When you undo the hydraulic pipes from the valve, take photos and mark which came from where.

Pipework makes harder to get at the priority valve
(photo credit - unknown)

Rebuilding the priority valve is straightforward though, before you do so, it’s worth sourcing two replacement ‘O’ rings.

There are two 'O' rings in the priority valve

I bought mine from a DS parts supplier, but the sizes are given in the parts manuals and you could source your own. Use nitrile for LHM.

Seal sizes

‘Stripdown and rebuild’ is perhaps overdoing it - it does a very clever and important job, but doesn’t have many moving parts to worry about. Perhaps that is why there are no English language instructions for a rebuild in the workshop manuals. It is covered at Operation 453-3 in volume 3 of Manual 583 - but that's in French. The next best I have found is an exploded diagram of a priority valve in the parts manuals. It’s usually just before the diagram that shows the hydraulic reservoir. Here it is from parts manual 562 - which is relevant to the age of my car.

Priority valve - best diagram I could find

It’s not exactly the same as what I found inside mine. And that's where that Manual 583 comes in handy.

Note the inclusion of part 4.......

That's more like it. The part numbering is different but Manual 583 correctly shows 11 parts - of which part 4 is the all-important spool valve!

I started by removing the cap.

Removing the cap

There is an 'O' ring recessed into the body beneath it.

The first 'O' ring

At the other end I unscrewed the long cover, which revealed  the spring.

Which was covered in black sludge...... Inside the long case (and buried in sludge) was an end piece for the spring - the 'spool stopper'.

There is another 'O' ring in a recess at this end.

A second 'O' ring.

With the spring removed, the spool valve is visible in the main body and can be carefully removed. It's a precision fit and should pull out smoothly - like a plunger. You need to be very careful not to drop and dent this, or scratch it. So don't be tempted to use any abrasive cleaners 

You will find attached to it is the thrust washer. I gave the parts a basic clean. It hadn't been immediately obvious because of the sludge, but there was a shim on the thrust stopper I'd removed earlier. You can see it in the photo below - between the spring and the spool stopper.

Nearly there. Lots of black sludge

The number of shims (part 5 in the drawing, part 6 in the photo) inside the priority valve may vary, as their function is to compensate for differences in spring strength by altering the lengths over which they operate, and so standardising it so that all units behave the same. I had just one shim in mine.

Everything was carefully cleaned more thoroughly. I used a wire wheel to clean the outside of the main main body and the end caps. I used a cotton bud with LHM to carefully clean the bore of the spool valve.  Parts were dried with compressed air.

The thrust washer that goes over the spool valve serves to hold the inner end of the spring and so, together with the thrust stopper at the other end, compresses the spring when everything is put back together. 

Ready for re-assembly

The thrust washer is held in place on the spool valve by a tiny circlip (part 4 in the photo fro manual 583). I left that in place.

Circlip (arrowed) and thrust washer on the spool valve

On the photo above, note the orientation of the thrust washer, and that it is on the 'long half' of the spool valve. 

The circlip prevents the thrust washer moving the full length of the spool valve......

Thrust washer on the spool valve

......and so in seeking to expand to it's natural length, the spring pushes against the spool valve.

I brushed some LHM into the valve bore in the body, onto the valve and around the thrust washer before carefully replacing the spool valve in the body. It's the 'short half' that is inserted. It should be a lovely, satisfying smooth fit.

The 'short end' of the spool valve gos in the body

A put a little LHM in the recess on the end of the body, and on a new 'O' ring....

New 'O' ring

......before fitting it.

'O' ring fitted in the recess

Next up were the spring, the shim and the thrust cap. If you look carefully between my fingers in the photo below, you can just see the shim already on the thrust cap.

Thrust cap and shim washer

I placed on the large end cap and began to screw it home. before I tightened it, I checked that the 'O' ring was correctly seated in it's recessed and would not get pinched by the cap and cause  a leak.

Check that the 'O' ring is correctly seated.

The 'O' ring did need re-seating, but that only meant carefully pushing it back down into it's groove with a screwdriver tip.

The French repair manual 583 says that the cap should be tightened to between 17 and 23 Nm (Newton metres). 1.7 and 2.3 m.kg. (That's about 12.5 to 17 foot lbs.) It's not a hex end, so I couldn't use my torque wrench as I don't have any spanner ends for it. It's also very long and even if it did have a hex end, i dong think a long socket would be long enough. Under the circumstances - and with quite a bit of leeway between the recommended torques, i just did it up 'tightly'.......

After lubrication with LHM, the 'O' ring and cap were similarly replaced on the other end. Now this IS a hex end screw, but there are no torque values given for it - so again i just did it up tightly.

Refitting the second 'O' ring

With everything now back together, I masked off the four hydraulic hose holes and then gave the assembled body a good going over with panel wipe before repainting.

'After'

I have been using BS226 'Brunswick Green' for repainting my hydraulic parts. It looks pretty close to the original. In hindsight I wished I had sprayed some on this part before I stripped it's original paint off  and so I could have seen the two colours side-by-side. The truth is that where I have put original Citroen hydraulic parts side-by-side the colours aren't the same anyway! So maybe that's not so much of an issue.

'Before' 

Replacing the Nylon Fuel Line

I've put in a new fuel line.

New fuel line in the engine bay

In anticipation of replacing the outer sills, I'd stripped out from the left hand sill, the wiring loom to the rear, the control rod for the rear height corrector (you can read about that HERE) and the  thin rilsan return hose from the rear to the reservoir. 

Height corrector rod under the sill

I left in place the run of four hydraulic pipes as mine had no joins in the middle and so were not easily removed.

Hydraulic pipes under the left hand sill

From the right hand sill I'd loosened the nylon fuel line and the rilsan return hose to the reservoir. I'd undone the brackets that secure the pipe to the face of the sill but it still passed through the bottom of the 'B' pillar. 

Fuel line passing through the base of the 'B' post

In removing the fuel tank and pulling the old and stiffened fuel line out from the tank and through the sill, the fuel line had suddenly 'bowed'  between the sill hole and the 'B' pillar........and suddenly folded on itself. I couldn't see any obvious damage but I'd read that this can be fatal as the pipe may then leak when refitted. So I decided not to take the chance and to replace it.

At the front end of the car the fuel line passes behind the right hand side suspension unit.

The fuel line and return pipe pass behind the suspension

So before I could refit the suspension arm I needed to refit the fuel line....I understand that efi cars have a fuel pump hidden in the sill - so presumably the fuel line is at least two pieces. On a carburettor car, the fuel line is one continuous length - from the bottom of the fuel filter in the tank, to a short rubber section that joins to it the fuel pump on the side of the block. The replacement fuel line I bought is 5 metres long. 

Where they pass behind the front jacking point, the fuel line and return hose are held in a long thin carrier bracket - held in place by a single bolt in the wheel arch. 

Bolt holding the carrier bracket

This is unbolted and pulled out towards the front of the car. Provided all the other little brackets along the length of the sill have been undone, and the black rubber 'pipe' connector on the rear end has been disconnected.......

The return pipe on the erm....'pipe'......

.....the pair of lines should pull right out.

Fuel line removed....

The bracket fingers were surprisingly hard to open. Within each one was a rubber collar. The bracket got a coat of paint and the the collars got a clean.

The new fuel line was coiled and springy and I grappled with it on the garage floor. Lining up the new fuel line to the old I worked out approximately where the bracket needed to be fitted on the new line - allowing excess hose at both ends for final adjustment when fitted.

New fuel line and carrier bracket

The fuel line follows quite a twisty route and it was not until I'd crimped it back in the bracket that I could do a trial fit. The fuel line goes in the top row of clips on the bracket.

Fuel line removed from the carrier bracket

The carrier bracket is slid into the hole in the wheel arch and bolted down. When refitting the black 'pipe' I though the return hose felt a bit loose and couldn't remember what it had been like when I first pulled it off. If it leaked, it would be hidden away behind the jacking point. Although it didn't look as though it originally had a collar around the thin end, I put a Ligarex band around the thin end to reduce the risk of leakage, and I replaced the clip on the fat end with a Mirex buckle.

Mirex buckle on the 'pipe'

Starting forward from the bracket, the fuel line goes up into the wheel arch corner where the suspension cylinder sits, before coming forward in the wheel arch.....

 ........and it then ducks down - held by a bracket - behind the suspension unit. 

The fuel line passes behind the suspension unit

From there it runs under the hollow of the beam across the engine bay.....

Fuel line running under the cross beam

......and emerges through a hole in that beam. The hole is lined with a grommet to stop the fuel line rubbing and wearing. I had a spare grommet for the where the hydraulic hose pass through the radiator bracket and it was just the right size for this particular job. I put some tape over the end of the pipe but otherwise left it loose in the engine bay.

Going backwards from the bracket by the jacking point, the pipes emerge under the front door hinges.

Pipes behind the jacking point

From there, the fuel line and return line run along the sill......

The pipes running past the jacking peg

.......and towards the base of the 'B' post.

Pipes at the 'B' post

Along that route, there are several 'm' shaped double pipe clips and rubber collars to hold the hoses in place. These screw into nuts welded inside the sealed box section.

After the 'B' post the two hoses go their separate ways. The return hose continues to the back end of the sill - supported by a couple of single pipe clips and joins another black rubber 'pipe' in the rear wheel arch. I've not joined it back up yet as I anticipate doing further work in the rear wheel arches.

The return pipe extends to the end of the sill

The fuel line branches off and passes through two offset holes in the sill to end up in the fuel tank gap - under the rear seat. There are grommets to line the holes.

Fuel line to the fuel tank

When I refit the fuel tank i will need to manoeuvre the fuel line again and I will need to be careful not to kink it again. For that reason I've only temporarily fitted the brackets under the sill at the back end of the car. I will need to make sure that I cut the line long enough so that it sits at the very bottom of the fuel stack  and doesn't ride up. I will probably cut and fit the rear end first and then adjust the positioning back towards the front. I've allowed plenty of spare at both ends at the moment.

There is also an outer sealing sleeve over the fuel line where it joins the tank. I will put a couple of Ligarex straps around the ends of this to make sure they are airtight and help the pump maintain suction.

Sealing sleeve over the tank end of the fuel line

Left hand Sill

I did similar work under the left hand sill. I refitted the long control rod for the rear height corrector - with it's replaced threaded end. I cleaned and replaced the nylon eyelets in the clips along sill that guide the rod. There are four eyelets and only three clips. the fourth eyelet goes in a hole in the 'B' post. The rod first pushes into the footwell from the wheel arch, and then needs to duck down (where the height lever would be) into the sill and out into the opening beneath. At first it was tricky to get the rod to flex enough to go into the sill but once it started it slid smoothly and was easily guided through the eyelets - which i greased in advance. 

Eyelet for the height corrector link rod

My friend Peter had made a new knuckle for the end. I liberally coated threads and knuckle with grease  - to prevent the rusting problems that caused it to snap on removal - and I temporarily put a split pin through the linkage. I will need to adjust the linkage once the hydraulics are rebuilt.

Rear height corrector linkage - temporary connection

At the front jacking point, the thin rilsan return pipe was attached to an old offcut of hydraulic pipe and (eventually) threaded forward, from behind the jacking point, and out into the wheel arch. It's held in place along the sill by a few arrow-headed rubber straps that join it to the run of hydraulic pipes.

Rilsan tube behind the jacking point

At this point i haven't refitted the rear wiring loom in the left hand sill. I will probably do that when I fit the new main loom behind the dashboard.