To maintain its market momentum, Gravity Power must complete the Series B round financing while completing final steps of product development. The final design hurdle is developing the piston seal. The GPM uses a stationary seal system mounted halfway down the main shaft. The seal must have low friction along the moving piston but must also hold against a pressure differential of ~500 psi and accommodate small piston diameter variation. In planning is a 2-m seal test to identify the optimum sealing material, followed by a full-scale 30-m test at full pressure and maximum piston speed. The seal development is a solvable problem; it may even involve using an off-the-shelf product.