The valves are the means for breath to evacuate the mask on exhalation. Within the valve (plastic component on the side of a mask) is a rubber diaphragm. The diaphragm is fixed to the valve and does not require any adjustment. The valve automatically works by opening on exhalation and closing on inhalation preventing unfiltered air from passing into the mask. The valve consists of the main body (on the outside of the mask) and a locking fastener (as can be seen on the inside of the mask) these two components lock together and so should not fall off. Please see the video for changing the filter as this will confirm how the valve locks in place. http://www.youtube.com/watch?v=b2XIXeCVmVw.
Valves do not affect filtration performance significantly but they do affect comfort as heat, water vapour and CO2 are released from the mask through the valves on each exhaled breath. If no valves are present, then heat soon builds up along with moisture and then begin to significantly affect filtration and inhalation resistance.
The valves only operate if the pressure in the mask increases; it then releases the exhaled air. If for instance, you are breathing through your nose, then the valves are not likely to open as there is no significant pressure increase due to the low inhalation resistance of the filter media, which is a good thing.