I have a question about the atmosphere poppet valve in this booster unit.

Can anyone tell me how freely this poppet valve should move?

Inside the piston of this unit:

The pivot arm has the vacuum poppet valve and return spring on one end. On the other end of the pivot arm is the atmosphere poppet along with a hold down spring.

If I press down on the atmosphere poppet end of the pivot arm, the atmosphere poppet moves downward and the hold down spring compresses. When I release the arm, the poppet position remains where it was and the spring remains compressed. What I don't know is if this is normal behavior. I would expect the hold down spring to return the poppet to it's original position. The poppet has a snug fit. It is clean, the tube it rides up and down in is clean.

With the engine idling, I measure 18 inches of vacuum. Step on the brake, the vacuum drops to 10 inches and the engine idle drops and the engine shakes a bit. I am thinking vacuum leak in the booster, but there are no tears in the diaphragm and the vacuum poppet appears to be moving properly. The piston o-ring is good, so I'm wondering about the atmosphere poppet. I removed the vacuum reserve canister from the circuit to eliminate it as a source of leakage as well.