Cavitation is when the impeller is spinning and frothing up the liquid that it is trying to pump. It isn't moving the fluid. That happens on water pumps or in hydraulic systems as well.
If it were me , i'd put the grapple back on and run it . If it's not then doing it i would be checking the plumbing of the new valve . Sounds strange it only started doing it when you removed the grapple . What is the other spool used for ? Your not using 1 spool for each action are you ?
Danny,
If you didn't notice this problem when you had the grapple off before, it will be a quick connector. Check all of them to see if they are seated. (Soundguy above) They can look like it and not be so you might even go ahead and take them apart and reconnect them. This is the most common hydraulic problem on compact tractors.
If it is tape, it would be very surprising.