leaking joint

Hi, Have used this forum a lot recently but this is my first post as i don't want something small to turn into a bog problem. Hoping someone can help me identify and rectify what needs doing, i have just moved into a new house and the boiler seems fairly new its a combi boiler and worcester...

Hi all, appreciate your help on this, I've a large new water tank in the garden. It has a 1" brass (female) outlet I screwed in a brass joint it leaked I unscrewed it, put ptfe around the thread, screwed it up again it still leaks What am I doing wrong?