The M-1, when in good order, has good power for a BB gun. Anytime you have very low power, the 'pop" valve is not functioning. This valve was a Crosman design that held back the air moving from the compression chamber to the barrel until a certain pressure was met and then it would "pop" open. This was designed to increase the power of the BB gun. When it fails, the velocity goes to poop.
I have not worked on these guns. Have no idea if any shop in the US does either. Spare parts for this model have never been commonly available.
Note: from my experience, functionality has little, if anything, to do with the value. Guys want these as part of their collection not to shoot.