Ok, let me try to explain what's going on with the Av and M modes when you meter with K, M or M42 lenses.
When you set the aperture ring on a K or M lens, or M42 lens in 'Auto' mode, the physical aperture of the lens remains at maximum aperture (e.g. f/1.4 for a f/1.4 lens), and is not immediately stopped down. There is a mechanism for the camera to tell the lens to stop down, and this is done in two cases: When you release the shutter to take the shot, and in M mode when you press the green button to meter.
So what happens if you use a K or M lens in Av mode? The camera will meter at maximum aperture, but when you take the shot, the camera will stop down the lens aperture down to whatever is set on the aperture ring (e.g. to f/2.8), before taking the shot. You would expect this to lead to the shot being underexposed because the camera is expecting more light (from an f/1.4 aperture) to come in but it's only getting 25% of the light because the aperture gets reduced to f/2.8... however I discovered something strange from my testings. The shots are correctly exposed AND I can see through the lens that the aperture has been stopped down AND the shutter speed is correct for the lens being wide open. My conclusion is that in Av mode, the shutter is actually released BEFORE the aperture is stopped down (or after it has reopened), which means that in Av mode, no matter what you set the aperture ring to, you will ALWAYS be shooting wide open.
If you use a K/M lens in M mode, the camera does not meter at all, until you press the green button. When you press the green button, the camera then stops the lens aperture down (to f/2.8) and then take a meter reading, then sets (and locks) the shutter speed and ISO. When you shoot, the aperture will be stopped down to f/2.8 and the shot should be correctly exposed.
If you use an 'Auto' M42 lens in whichever mode, no matter what you do the lens aperture will always be stuck at f/1.4 because the camera is not an M42 camera and doesn't have the necessary mechanism to stop down the lens aperture.
If you use a 'Manual' M42 lens, then whenever you turn the aperture ring, the lens aperture will open or close depending on the setting on the aperture ring. If you set to Av mode, then the camera will continously meter correctly all the time and adjust the ISO and shutter speed. In M mode, the camera will not meter at all until you press the green button, then a meter reading will be taken to set and lock the ISO and shutter speed. No stopping down of the aperture is done so you will not hear the "shzz shzz" sound. In effect the green button acts like the AE-L button.
Conclusion: If you are using K or M lens, use M mode with the green button. If using M42 lens, use Av mode.