SSC JE Mechanical Engineering 2018 Set 5 Paper

For a siphon to work satisfactorily, the minimum pressure in the pipe as compared to vapour pressure of liquid should be more.
If the minimum pressure falls below its vapour pressure, the formation of vapour bubbles in those regions(usually at the pipe top) causes decrease in the siphon effect(i.e. Weight of the pipe liquid below the reservoir > weight of pipe liquid above the reservoir).