It is expected that the ground water upon returning to the static water level will push the piston up resetting the system. To verify the return capacity 

Assuumptions

• The standard diameter for water wells typically falls within the range of 1 to 1.8 meters, so let us take 1.8 meter diameter well

       r = 0.9m

• The static water level in a well typically falls within a range of 2 to 5 meters below the ground surface, so let us take 5 meter depth well

      h= 5m

• The coefficient of friction for standard piston seals generally falls within a range of 0.2 to 0.8, so let us take the highest for safety which is 0.8

      µ = 0.8

 At static water level the summation of all forces excluding the weight of the person should be zero

Therefore as long as the mass of the piston is less than 7,063.46 kg for standard well, the ground water will push the piston plate. The weight of sheet metal used for a piston depends on the material and thickness. For common materials like steel or aluminum, the weight per square meter (kg/m²) varies with thickness. Steel sheets weigh between 7.85 and 10.21 kg/m².

Since the well diameter here is 1.8 meter giving 2.5 m² and taking the weight 10.21 kg/m² for safety,  the mass of the piston plate will be 

m = 2.5m² * 10.21 kg/m²

m = 25.525 Kg

Which tells us the design is safe.