Ok, given the recent threads on the lack of availability of new heater matrices I wanted to post up my experience of flushing my "not quite working at optimum" heater.   Firstly I should say I have a perfectly good very clean low mileage Supra with a piping hot heater to compare my other slightly more worse for wear one.   Symptoms of the problem were the usual poor heat output, particularly at idle, but it was reasonable at cruising rev's. A cold/luke warm return/out pipe from matrix, the ret