Though I had the tool for years it managed to walk sometime before the last two times I needed it. Both times I needed it to get to a stuck piston. (single cylinder misfire, due to no fuel). Both times I got the valve out by rocking it back and forth with two small screwdrivers. The valve is only held by a teflon o-ring seal.
I seriously doubt that the valves are the problem. Fuel being incompressable it would have to go somewhere if the pistons were going up and down. The pistons are pushed up but sprung down; sort of like the intake and exhaust valves. I suspect that the fuel evaporated and the residue has stuck the pistons up.
Another posibility is that the shut-off solenoid is powered. (not all pumps have this solenoid.
If you can get a valve out crank over the motor and see if the fuel pistons go up and down.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician