I don't know about the Caroma "Cosmo" but our old Caroma built up scale in the valve over the years and also started leaking.
Our plumber simply replaced the whole float assembly with a "new, improved" float system. It worked fine... until the first time it was used after he left, when it started leaking even worse than the old one it'd replaced!
I'd kept an eye on him whle he was working and noticed he'd tossed the instructions, etc. in the bin as he was working... without even glancing at 'em. I did some scavenging and had a read and this "new, improved" float was designed to work on high-pressure mains. In black and white it stated, quite clearly, that it wasn't suitable for low pressure lines.
In short, it might be worth your time to do some checking and make sure that the "Cosmo" is rated for your water pressure...