The Repair Job (took about 2h, really not bad)
I'll embed photos and also attach them at the bottom, as I hate finding vehicle repair guides where all the pics have been dropped by the host. I'll attach in the same order that I embed them here.
The repair kit looks like this (check if your current one has 4 or 6 bent over tabs on the back and make sure to get the same replacement):
Get everything out of the way so you have access. Air intake, coolant tank etc.
There is probably a way to remove the filter from the heating part on the bottom, and hence remove the entire unit, but I couldn't find out how to without being forceful and didn't have the time to risk damaging things so I did the entire job with it still attached to the coolant system in the engine bay. this probably made it a lot more awkward than it needed to be.
Be careful not to lose any of the washers when you remove pipes etc.
So, bend back the metal tabs on the current primer. I mainly used a screwdriver to do this.
Once you get them off the whole front will kind of ping off as there is a spring inside. Be careful to catch all the parts as there are a few layers.
Once that bit is removed, the inside will look like this (minus the rust, hopefully):
So, the cause of my problem had been found. There is a black metal ring that holds some bits in place under the spring. Tiny amounts of water in the diesel has caused the bottom part of the ring to go rusty, and those bits of rust had opened up a gap in the seal. The more I'd pressed the primer, the wider the gap had become.
I removed the ring and over components, and cleaned out the rust and mess (also found a small leaf, of all things).
I mostly used kitchen roll, and a bit of scraping with a screwdriver for the tougher parts. Be careful to clean and wipe thoroughly after using the screwdriver so you don't create any little metal shards that could bypass the filter and get into the fuel pump or engine.
End result wasn't perfect, but certainly good enough. Make sure the surfaces are all flat where the seals will sit so there is no leaking:
These are all the parts that came out of the primer. A few bits had rust residue, so everything got a thorough clean. Note that the repair kit only has replacements for a few of these pieces, so you'll need to reuse most of them.
Close up of the offending metal ring. I cleaned it very thoroughly and lightly sanded to remove tougher bits:
The white plastic layer with the valves got a good cleaning. The kit came with new valves so I figured why not, and used them. However it wasn't necessary for this job so I haven't included any pictures. You just pull the old ones out and push new ones through, but do one then the other rather than removing both to make sure they go in the right way round!
I inserted the black metal bit the other way up so the rusted bit was out the way of any water that did get in, so it should last another 164,000km now
This is how it should look by this stage:
I had a small cup of diesel and made sure to give all seals a diesel bath before putting them back in place. There is a little bit of metal that sticks out that will prevent you putting anything back in the wrong way round.
Next up reassemble the spring/button assembly. The order is:
Metal cover, button, big seal, plastic lid thing, spring. I turned the spring around to put the cleaner half on the bottom (top in the photo). Again, diesel for the seal.
To put it back, you need to get a decent bit of compression on the spring and seal. I folded one metal tab using a pair of pliers and hooked that on to the main unit to keep it all in place. Then used that as a hinge to press the assembly down.
Make sure you don't bend the tab too low down or the other side will sit too high up. Bend the tab about half way between the end and the base.
The next part sucked, and was very fiddly. Mainly because it was all this attached to the engine and took one hand just to hold it.
I didn't get any photos as I couldn't put anything down to pick up the camera.
I managed to get the other tabs down part way with a hammer (just enough to overcome the spring and make the assembly stay put), and then used a pair of pliers to fold them the rest of the way.
For the first tab opposite to the pre folded one, I ended up using a small woodworking clamp to hold the assembly in place while I hit the tab with a hammer.
Once you have the first two tabs gripping, check that the assembly isn't uneven compared to the main unit. If it is, remove and try again.
Once that's all done it should look like this:
The hard part is done! Now we can put everything back together again.
The screws that attach the fuel lines to the primer and pump have certain directions that they will work in, because the flue flows through them.
Score the outside of the bolt so you know where they need to end up.
Now, reattach the line from the tank to the primer. Hold a glass by the outlet and pump that button! keep going until you get a strong blast of diesel with each press, and then press a few more times. I think there might be a bleed screw on top, but I didn't seem to need it.
Then reattach the line to the fuel pump, and undo the connection to the pump. Hold the now unattached end up in the air so that it is the highest point, and make sure that there are no places where the line goes up, down, and them up again, and pump the button again. Keep going until you have a good strong blast with each press, and then a couple more for good measure.
Reattach to the fuel pump, again marking the top of the bold. This one only has a hole on one side rather than both opposite sides like the last ones.
Once this is done, the only place where you can have air left is in the fuel pump itself.
To bleed this, loosen the nuts by the injectors again as mentioned in the previous post. Then get someone to crank the engine with their foot on full throttle. Keep going in 10-15 second runs with little breaks so you don't burn out the starter motor.
When you see a steady stream from a line, stop cranking and tighten up that nut. That means all air is gone from that line.
If you've done 2 or 3 but are still waiting for the 4th the engine may start coughing and trying to start, that's ok just carry on.
It will probably be the two nearest ones that will be steady first, as they are the two bottom outputs from the pump.
I took a video but it's not very clear as the engine is shaking a lot. The nearest one is clearest, but all should look like that:
In my case all lines started pouring pretty much immediately, so after filming I stopped cranking, tightened them all up, and gave it a test.
The recorder missed the second cranking, but it went almost instantly and with no easy start:
Everything worked perfectly after that! I took it for a 10 mile drive to be sure and it ran wonderfully.
I upgraded the headlights, cleaned everything, did a wheel alignment, fuel treatment, reconditioned the leisure battery, touched up the paintwork, and made everything smart and tidy.
The next morning it passed the French MOT with no required work, and by that afternoon it was sold to a very happy young couple who planned to live in it for the next several years while they explored Europe and searched for their calling in life.
All in all a big success and my thanks go out to the willingness of forum members to offer their help and experience. Plus a friend of mine who pointed me in the right direction several times!
Specific mentions to Dancingdad and Ours2012,
A big journey for a little van (dubbed Hugo by the previous owners)
If there's anything I can add or change to improve the post then let me know and I'll get on it!
Mods: The Youtube embed tool doesn't seem to be working correctly and I'm not very familiar with the forum tools, would you be able to get it working correctly? Thanks