Valve seats

[DanoDMano]

I have a question for you motor guys who have been giving me a hand through my project.Today I started replacing my valve seals with the umbrella type. I have the engine on a stand, and the heads are still on the block.
When I pressurize the cylinder to hold the valve up so I can remove the spring I am getting alot of blow by from where the valve seats. On a couple of the cylinders it has been bad enough to make my compressor (5 gal) cycle constantly.On one cylinder in particular I could not hold sufficient pressure to force the valve to stay up!
I am contemplatin pulling the heads and lapping in some new valves.

What do you think? Should I just remove the heads and replace the valves? Should I have the heads machined, if so what kind of cost am I looking at?

I did not plan on going through the lower end, and am also wondering what to expect if the heads are done and not the lower end what may happen to the rings?

Any help would be greatly appreciated as usual!!!