Valve Stem Seals On Vehicle Replacement w/o removing the head

[JPERL]

I have heard that you can replace valve stem seals without removing the head but I wonder how you keep the valve from dropping into the cylinder.

[Burntboot]

I've done it (not on a van mind you, but the procedure is the same)

Remove a spark plug and with the piston 1/2 way up, feed a nice (long) soft rope into the spark plug hole.
Make sure you leave enough sticking out to grab hold of.
Rotate the piston up till it compresses the rope.
Once the seals are changed, rotate the engine backwards a wee bit and pull the rope out.
Move to next cylinder.

I have seen people do it with compressed air and it works like a charm, until it doesn't.
If the compressor bleeds down the valve can fall.
If the pressure slowly pushes the piston back down, the valve can fall.

The rope method solves both of those problems