Hydraulic lash adjusters can eliminate clearance between components in the automotive valve trains. A two-mode dynamic model for the adjuster is derived by including check valve motions at two operating modes. The oil refill mechanism controlled by the check valve is experimentally identified. The oil compressibility in the oil chamber and the oil leakage through the annular gap between the plunger and the body are also considered. It is shown that motion of the adjuster plunger must satisfy a third-order, non-linear differential equation of motion. The differential equation is numerically integrated, and the experimental data in leak-down test provided by the manufacturer is shown to be in good agreement with the model. The annular gap is found to be the key factor in controlling leak-down speed, and the effective orifice diameter is shown to be the key factor in describing the pump-up phenomenon. It is also found that one percent air entrainment in the oil chamber of the adjuster seriously compromises overall adjuster rigidity and reduces the natural frequency of the adjuster almost 50 percent.