The easiest way to make a piston with different bump and rebound is to have a piston with 4 holes in it with a reed valve on the top covering two of the holes. Going up the valve would be forced shut so only two holes are open, going down the valve opens and all four holes are open.
Its as simple as a disk of rubber clamped to the top of the piston using the c clip with only half of the holes in it that the piston has.
Easy peasy.
