Personally, i don't like the sealants Keith. there's lots of small water passages that can block and the mechanical seal on the back of the pump impellor could be compromised.....
A safer bet would be to have the system pressure checked, either in situ or have the rad checked off the bike.
The expansion tanks can cause problems. They are getting old now and the plastic can harden and crack. Can be quite obscure when cold but opens up when hot and under pressure.
On the bleed screw, try a new copper washer and/or a dap of sealant on the threads.