Well this is going from bad to worse

Mr Spanner had van problems (Ford ECU packed up) so only got here today at lunchtime - got the fitting out only to find that the actual leak is the main aluminium housing having a crack adjacent to the fitting

Needless to say they are like hens teeth, and every machine being broken currently seems to have been robbed already. JCB want about a grand for just the block without any of the gubbins.
Theoretically it should be possible to reverse engineer the existing block and make a copy, but in practise there are so many interconnecting threaded bores, most with a seat cut at the inner end for a relief valve or pressure setting valve that the chances of success are slim. It runs at 3000 psi so needs to be sound.
Talking to specialist breakers this seems to be a known problem, especially in hot weather. When did it happen - hottest July day on record

At the moment my hope is with a breaker in Wales who thinks he may have one of unknown goodness, but still £250

It's tempting to strip it down and attempt a repair using one of the low temperature aluminium solders - probably mill a channel with a ball ended cutter and fill the trench. (I think TIG welding is likely to distort things too much).
If the breaker turns up trumps, I'll fit his, and if it works use it, and if not attempt a repair on one of them.
In all not a good day - the replacement tracks, idler sprockets and drive sprockets arrived yesterday so I'm in too deep to stop now - it's a bit like the addicted gambler who has to double his stake every time he looses to keep up with his stake
