Consider picture below where a force applied to a water by a piston and suppose the gauge reads an arbitrary pressure. If I lock the piston to its position and remove the force applied to piston, will the pressure on the gauge read the same? Consider that the manometer is below water level so it is filled already.
Assuming that heat flow into/out of the water is negligible, then the water remains at the same pressure. The pressure of the water is determined at a given temperature and volume by an equation of state, and in the situation you're describing the temperature and volume are remaining the same.
Note that if you "lock the piston into its position" then the locking mechanism will end up applying the same amount of force on the piston as was originally being exerted on the piston by the external agency. It has to, otherwise the piston would move.
When you lock the piston and remove the force(ie pressure) in the process of locking the piston, you are still applying the force required to hold the piston in its current volume, and if the piston is unable to lock itself of course volume and pressure will change. Assuming the lock is strong enough, i think the pressure remains the same. (all this assuming walls are adiabatic)
