Has anyone come up with a better setup to the above ? I'm always having problems with caps not sealing correctly even when buying the better ones.

The cap has a pressure relief valve in it. Are you sure the problem is leaking caps and not a too highpressure in the system?

Other than that you could try fitting a new expansion tank as yours could be bad. I dont think it is normally a problem

My 1993 carb 1.1 K series makes a vague whistling noise from the expansion tank cap after a run, that lasts for a long time. The cap is seated correctly so the noise must be the pressure valve in the cap. I have replaced my coolant and it is at "max" and the level does not visibly go down so I assume that at some point, when enough steam has escaped, it'll stop.