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'