These, as explained earlier, are most likely pressure / flow control valves of some sort.
1) for cushioned slew protection (cross line relief),
If the crane boom is knocked sideways (or when slewing the crane a heavy swinging load tries to keep it turning when spool is suddenly closed), the spring controlled relief valve will resist oil flow (and hence movement of boom) up to the preset pressure only. Above that, the valve will lift, allowing oil to flow possibly from A1 to B1, and thus the boom will be able to swing sideways a little, rather than bend / break something. One valve controls flow A1 to B1, other valve controls B1 to A1. In order to work they require a certain pressure in the oil line from spool to build up before allowing normal flow to service, hence apparent blocked line A to A1 when using airline.
2) for slewing lock and movement precision (in line)
Another possibility, is that these are intended to act as a pressure restriction for the slewing motor movement. The slew motor will not behave like a simple slew ram (which will instantly stop when spool is shut) - instead it will slow to a stop, and can also allow creep slowly if crane is not absolutely level, thus giving an imprecise response to slew commands. By requiring a pressue valve to be overcome before oil can flow (e.g. from A to A1 and from B1 to B), it may be that the motor (depending on type :dunno:) becomes more like a ram in response, i.e. much crisper movement and resistance to creep.
3) hose rupture check valve (in line)
If hose bursts e.g. between pump and spool valve, the boom might fall sudenly. The valve (in cartridge form behind hex plug) permits a certain oil flow but if greater flow occurs then it "locks out". These valves also require a back pressure to work correctly, again an airline might not be enough to overcome this minimum setting to allow flow.
4) pressure limiting valves
Where presure to service should be limited below relief valve setting. Any excess is dumped to tank via cross line. Unlikely to be these, as no back pressure required, so air flow would be A to A1 no problem.
As to trouble with main spools, what happens to air flow when spool is operated in other direction?
?Could be spool and housing is assembled back to front.
?Might be wrong spools in block (e.g. single acting spool in double acting block).
?Might be a Closed Centre unit (where pump builds up max pres. in spool valve line and then stops pumping, when you operate a spool the pressure drops and pump starts flow - older John Deere tractors (1970's / 80's) were all like this, but not commonon application on truck cranes).
I can't help anymore without playing about with it - why not ask Santa for a new spool valve assembly ;-)