We have a downstairs wc and a main wc/bathroom directly above it. Yesterday I flushed the downstairs loo and ALL the water syphoned out. I flushed again and it backed up-then syphoned out again. So........I assumed blocked drains. Just had manhole cover off.....no problem .....everything clear and flowing freely. Upstairs loo flushes fine. Spoke to a mate who thinks there may be a blockage in the stack between the 2 WCs. Said blockage may be preventing air being dragged down, therefor upstairs is fine while downstairs isnt. Any WC/FLUSH experts on here ???