Check Valves, are also referred to as  ball check, ball and retainer, one-way valve, ball valve and check ball.

They are used in applications where fluid (transmission, steering, braking or cooling) needs to be controlled to fully flow in one direction with partial or no flow in the other.  These devices are a two-piece assembly that has a precision steel ball confined in a retaining cup with openings at both ends.  There is a conical seal area at one end of the ball retainer and it is open at the other.  

When there is pressure on the ball forcing it to the seal area, it does not allow fluid to flow.  When the pressure in the system is applied in the other direction, the ball leaves the seat, and the fluid can flow.  This small product is very effective in performing this function, and it is also an inexpensive solution to a fluid flow problem.