My job is supplying parts for Volvo ADT's and i can hand an heart say you won't find any repair kit for that bit, i have tried for years :-( we can supply the ball bearings but you will need a close approximation of the spring, the worse bit will be you will have to manufacture your own socket and key as these are normally corroded as well as the socket and guide sleeve on the big spring, from memory the part you are on about is what hold the valve in the "float" position, if your only using it every know and again i wouldn't bother trying to fix it just remove that part and it'll be fine, if however you are using it a lot on a haul road it won't do your hydraulics any good, I wonder if you can make a bracket in the cab to hold the lever in a float position?
** update** i have taken a close look at an old valve block, they were made by VOAC monsun-tison, which was originally a joint venture between Volvo and Atlas Copco, Parker bought them out back in 1996 just as Volvo will have been thinking about phasing out C series trucks, so Parker probably didn't even bother continuing them. they also have nothing like it listed on current web pages.
**** further update**** i have managed to disassemble a cap and that part ( broke the cap to do it) the balls are 5mm and a spring looks to be 6mm o/d 3.5mm i/d and approx 12mm - 14mm long mine are corroded on the end, the problem is knowing the strength of the spring
