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!