I strongly endorse obseen16's posting ....I tried leak sealer a year or two back - hoping to overcome leaks in copper pipes buried in concrete since the mid-seventies (it didn't work - the leaks kept coming - one by one, the solder joints gave way!) I've now bowed to the inevitable, and am in the middle of replumbing all my rads from a new loft circuit. I've just spent a very unpleasant few hours dealing with a blockage of my feed and expansion pipe, following a system drain and refill. The blocking matter was a glutinous mass, sitting on the bottom of the F&E tank, the consistency of thick, cold custard - come to think of it, thick snot about covers it! Because I didn't realise that the system had only partially refilled, the boiler blasted steam back up the F&E pipe. Mercifully, no damage appears to have been caused, but some of the muck got into the new loft circuit, preventing the rest of the system from filling. Eventually the head of water in the F&E tank slowly pushed the muck down into the boiler, and I assumed that the turbulence in the boiler, coupled with the pump action would break up the sludge, and the system now seems OK. I'm not confident that the presence of this stuff in any system won't "as it says on the tin" block air vents and air bleed screws. I'd say that if you've used leak sealer in a system with an F&E tank, ALWAYS clean out the F&E tank before refilling. It won't be pleasant job, but it can save you a lot of time and trouble!