Took a standard cap:
cut away the valve, removed the rubber seal (but kept the majority of the black section) and dilled a hole in the yellow cap. Then cut the valve out of a bicycle inner tube (with car valve)
turned the compressor down to about 1.5 bar, filled the header tank, stuck the cap on, applied pressure, open engine bleed valve, then rad valve. waited until all the bubbles were gone and coolant ran clear and shut them off. topped up the coolant and started the engine and got instant circulation into the header tank,
Its not 100% as i still have a few airpockets in the rad, but it just needs to be driven now i think.
The system works, i may need to improve the process.
