spent  all day trying to fix a problem with me mums washing machine  instead of being out on the bike 
 
  frankly i'm baffled 

 so if you can picture this can anyone suggest anything ? 
 there is a waste pipe behind the machine leading out to the drain  via  the sink waste pipe  at the machine  end there is a u bend and an upright pipe  which the waste pipe from the machine  goes into  ok? .
 waste water  was coming out of this pipe when the machine emptied itself  so i used caustic soda followed by boiling water and the water seemed to flow fine  even put the garden hose in to flush any rubbish out  but when the machine waste pipe was put back in  water  just  came pouring out again  tried again and  it was ok  then again and the water  came pouring out 
 
  when the machine empties you can hear the water exiting into the waste pipe and then backing up the  upright pipe and overflowing out  even though there is no blockage in the pipe anymore , is it possible there is an airlock or air vacumn somewhere  stopping the water flowing out ?. 
 sensible replies please 

 my mums kitchen is  very wet 
