Valve seal replacement

I have a leaking valve seal any tips on doing them while the heads are still on? I heard you can fill the cyl with air to hold the valve, how much pressure would it need?

 

On my 430 i removed the valve covers and the rocker shafts so the valves are closed. I removed the sparkplug(s). Then i made a connection to fit the sparkplug holes which i connected to a compressor to fill up the combustion chamber with air. Make sure you keep pressed air in the chamber, you don't want to have a valve fall on top of a piston and remove the head.

 

By the way, i don't remember how much psi but let's say 60-90 psi.

 

i used nylon rope in the cylinder
works like a charm

found a video online:

 

100 PSI would be safe.

 

I've used a section of bungee cord in the cylinder on several occasions. Works like a champ.

 

Remove one l plug at a time on the cylinder you're working on. Too much pressure and the engine will try to turn over and removing all the plugs will only make it worse. You only need enough pressure to keep that from happening.

 

I use an air hold adapter (I think that's what its called) looks like the hose for a compression tester, or leak down tester. I just plug it into my air hose, my compressor turns off a 125 lbs and back on at 90 lbs.
Just make sure your compressor stays turned on