[axsaxoman]

remove rad cap --stuff a 2 litre coke bottle in with a rag , with bottom cut off
fill half way up --open bleeders =close again when water present and run for few mins and stop +then run again --if its air locked that will fill it up

your rad is lower than engine and heater so just opening bleeders won,t do it
needs to have head of water higher than it to get all air out
usually you hear grurggling noise from heater if it has air in it
,could also be heater matrix is partially blocked
it is 15 years old !!
but bleed correctly first