Don't use the piston to push it up. If its not aligned, microns, you'll snap it and scar the head, bore, and piston head. Use a magnetic reach that can get in the hole or, use a probe through the spark plug hole. When changing valve seals, use a compressed air adapter, in the spark plug hole, fitted with a gauge and valve. These are pretty cheap at most auto parts stores. Do not exceed more than 3/4 compression rating of the engine cylinder, or you risk blowing the head gasket (at which point you might as well wait until you pull the heads, to do the valve seals) with the compressed air, this will hold the valve in place and allow you to change the seals with no problem. Make sure you lube the new seals with clean engine oil before installing. I just did this on my 500 SEL, and it works like a charm.
I hope this helps.