I did one umbrella seal on 1 valve only....lol and no I'm not kidding.
A couple of years ago you might recall I lost compression in my No. 7 cylinder (the one at the back nearest the driver).
I thought I had burnt and exhaust valve but in actual fact the spring had broken and slightly set the valve open...so no compression.
I used the 'soft cord' approach where you slowly fill/thread the cylinder up with some soft cord then move the piston upwards...thus forcing the fill of cord up against the bottom of the valves...keeping it/them up.
Oh, don't forget to keep some of the cord outside the cylinder so you pull it out afterwards, lol
Then I used an overhead spring compressor to clamp/set the new spring and voila'...the new spring was on as well as a single new umbrella seal.
It all did the trick...at least on that cylinder.
Trouble is, some of the other valves suffer from oil leak down so I burn some oil on start up....so maybe I would be well suited to add the umbrella seals to all the remaining valves.
To my understanding it only really works on the exhaust valves....correct me if I'm wrong.
All good
Coley