Custard has a low level suite with boxed in cistern. Toilet not flushing properly. Removed siphon and checked flapper valve, all okay so changed siphon. Toilet still not fushing properly. Toilet only filling just over half full, if I hold down the ballcock so it fills to about 3/4 then it flushes more or less okay but does only empties just below half full during the flush, so not maximum flush. It is difficult to bend the arm on the ballcock to get it to let in more water but even if I did then it would flush but still not to maximum. Appreciate any thoughts.