Pilot goes out despite replacing thermocouple?

Hello, in my TA 20/30 the pilot goes out when the main flame is switched off by the thermostat. I have replaced the thermocouple and brushed the quite rusty pilot thingy but it didn't help. I noticed, cleaning (the slightly sooty) thermocouple with a brush wire helps for a day or so but then it misbehaves again :-(( The entire heating system was professionally flushed a hear ago. Please advise.