The MC4+ Rapide Leak Sealer 300ml is a high-performance internal leak sealer for central heating systems that is both powerful and fast acting, and the high concentration guarantees a strong seal. Because it flows around the entire system, it gets to work even in hard-to-reach places, including inaccessible leaks and weeping joints. With its quick-dose bottle design, 20 seconds is all it takes to add it to the system.