That's odd, if they have been done and don't leak immediately, then chances are they won't for another 10-15 years. I always pressure test them overnight and if they haven't leaked in that time, they aren't going to.

Must admit I hadn't tested that thoroughly when I overhauled the block, so could have been leaking some time...barely drove it afterwards...

I had the last O-Ring set from X8R, but heard some comments that they are not always perfect anymore. Does it make sense to redo the overhaul? How are the experiences with 4x4airseals?
Or could something else be the problem?
The square seals unter the solenoids are just for dust, aren't they ?

I bought a job lot of valve blocks and have worked my way through some of them. Initially I used the X8R kits but saw the same complaints you have obviously seen and I did have a couple of problems with the very thin O rings around the base of the solenoid plunger, so tried the 4x4airseals kit. I measured the size of these O rings and found them to be identical to the X8R ones. The 4x4airseals kit comes with a small sachet of silicone grease that X8R don't supply too so have used them since. Not all of the solenoids have the square seal under them which is why you only get a couple of them (in fact the latest 4x4airseals kit I have got doesn't include them) but as you say, they are only to keep dust out.

To be quite honest, just recently I have stopped taking the valves themselves apart to replace the solenoid plunger O rings. I have found that when you take them apart, the plunger will rotate so the groove worn in the rubber seal on the end then no longer lines up with the piece it should seal against so you actually create a leak that wasn't there in the first place.

Got the exhaust fixed (again). Back to smooth and silent!

Since my EAS valve block was leaking at the base of the solenoids I decided to di the O-ring job once again. Also the thin valve stem ones, I could feel on some that they had creases and bumps on them even though I changes them a couple of weeks ago.
System is still leaking air quite rapidly (abot 30 minutes for one corner to go all the way down) but this time from the cables that come out of the solenoids.
From where does that air come from ?? The valve stems again ? But wouldn´t the extra o-rings stop the air leaving the whole solenoid...?
I attached a photo and short clip here:
"https://c.web.de/%40337510546125364120/YIGJy3S8TNMUx5Vy6QMRdw"
I have overhauled the block three times now and still it´s leaking....! I don´t get it....
Had an appointment fpr MOT today but I guess that´s off. If the car sinks down while looking at the underside that´s not helpful.....

Your leak could be from the base or from the solenoid tube itself so it looks like you will be taking them apart again. I test them by connecting a compressor to port 6, where the reservoir would normally go, and opening all 4 corner valves and the inlet valve. That puts pressure into each outlet so I can bench test and look for leaks by leaving it overnight. If the gauges haven't dropped after 12 hours or so, they are good. If they have, then out comes the soapy water (as the weather has been a bit iffy recently, I've got 4 valve blocks now all ready to go for anyone that needs one). Doing it like this also checks all of the channels in the driver pack too. Then I can apply power to the 4 corners and outlet valve to let the pressure out.

Note that I test at 12-13 bar and as the system runs at a maximum of 10 bar, if they don't leak at 12, they aren't going to at 10 bar!

What can I do differently ? I did the Block three times now. I could sand/grind the rubber part of the plunger so that the circular groove is gone so it might seal better, otherwise I don't have any better ideas. The O-rings I renewed last night were all put in carefully and everything was very clean...
Is it possible to buy the plungers and/or NRV valves separately ?

Take the coils off the solenoids and see where the leak is actually from. I've known them to leak around the plunger seal, out from the bottom where they attach to the main block and even between the two halves of the valve itself. It's not as easy with the block in the car, particularly if the leak is on one of the bottom ones, much easier on the bench but that involves plugging the outputs so you can put pressure into it.

I bought a job lot of valve blocks and have been working my way through them, so could supply you with one that is all done and known good. Drop me a PM.

I don’t think you can buy the NRV’s. I needed to disassemble 3 valve blocks to find 3 good NRV’s to assemble one valve block assembly.

You're right, I've got 6 or 7 that I don't consider worth using as they are badly worn. Most of the 12 valve blocks I bought don't appear to have had much use so the NRV's are in good nick. They came from a company doing off road conversions, and the main problems I have found with them are split diaphragm valves and bad connections in the driver pack to solenoid plug so not all channels were working. I leak test them initially and if the valves are leaking badly, put it to one side and move on to the next one. Any that appear to be good, I'll re-tension the sockets in the solenoid plug, replace the diaphragm and check the pressure switch to make sure that is working. Then replace the O rings and do another final leak test before declaring it good to go. So from those with leaks, I've probably got a few spare valves that weren't leaking.

Finally got my MOT done yesterday (the german equivalent), yay !

Luckily the tester is a friend of mine (but very thorough) so the leaking eas valve block officially wasn´t an issue....He knows that if I say I´ll take care of it, I will. I just had to start the engine a couple of times in between when I saw that the car started sinking down....!
Thanks to Richard that problem will be gone soon too !

It was running a tiny bit too lean to pass the emission test but holding the exhaust pipes closed with paper towels did the trick....
Only advisories were Oily engine, rusty exhaust from the outside and brake disks a bit rusty (it sat for a year)

Glad I got it on the road again, while it sat waiting for a new flex plate I wasn´t sure if I would have the stamina for it...

I looked at it out of the window an saw her sitting down on her bumpstops and thought - yep, no more excuses!