Here are some notes I made when curing a 766 that would not pump up. As far as I could see the pump seal was OK so I needed to delve a little deeper. My problem was the O ring on the valve body. I did not strip the valve itself.

CROSMAN 766 STRIP

Taking apart.
Remove barrel band (2 halves, single small screw goes though band)

Remove barrel band ends: plastic, lifts away.

Knock out the pin holding the pump lever to the pump tube.

Remove pump lever with pump attached.

Remove barrel: hex key on top of action.

With the breech block on its side, screws uppermost, stock to the right, remove the 4 screws. Keep top plate in position while doing this.

Gently prise the top plate off the bottom plate. Take care the pump handle plastic grip holder, black plastic sprung laden bit on left, does not ping out as you release the top plate. Top plate may come away with the trigger and safety catch or may leave trigger and safety in the bottom plate. Either way the trigger safety spring may become detached from its location on the safety. Do not worry. Remove trigger, spring and trigger pin.

Note position of trigger sear components.

Lift sear away, taking care not to lose the large spring that acts on the sear bar. Lift off the long spring and plastic peg that acts as the BB follower from its channel.

The valve housing is held by a screw to the left of the valve housing. Before releasing this, note the brass component at the top of the valve body towards the left. This has a small spring over it. Release and remove the screw holding the valve housing but hold the housing in position. Now gently lift the housing away to the right, catching the small spring on the brass piece and also noting that the striker piece and a black plastic end body will be released under spring pressure on the right hand side. Just go slowly and this component will not be forcibly ejected. The brass valve body will slide out of the air cylinder. If it is an easy, loose slide then inspect the O ring as it will most likely be worn.

If needed, replace O ring.

Clean all parts, especially the air cylinder.

I have not dismantled the valve.

Rebuilding
Reversal of above but these tricks may help.

Assemble valve body and housing into place in the air cylinder. Have the brass component in place but do not fit spring yet. Take care that the striker and its spring and the black plastic end cap are located correctly. Replace the screw to hold the valve housing.

Carefully replace the small spring over the brass component. There is just enough room to wiggle this over the component; it stays in place against the bottom plate.

Refit trigger sears in place and slip the sear spring into place.

Refit the pump handle retaining spring and catch ... take care it has a habit of falling out.

Refit the trigger and its rod with the safety spring under the trigger and the hooked end pointing up. At this stage I found it best to hook a small bit of thin wire over the long arm of the spring and feed it towards the trigger such that when the top plate is replaced it protrudes out of the trigger guard.

Replace top plate: a bit fiddly to do this and keep pump handle spring in place but persevere.

When located, pull on the bit of wire to pull the safety spring long leg downwards ... look through the hole for the safety catch to see if it has cleared the hole. When it has, feed the safety catch back into place and release the spring. Pull the bit of wire out.

Replace 4 plate screws.

Refit barrel, and pump etc.

Job done.