Since reeds are normally constructed in pairs, one for 'draw' and one for 'press' of the bellows, valves are fitted to stop the flow of air through the idle valve while the other is playing. They are a very important part of the tuning process and have a major influence on how the reeds sound, air tightness and reed sound attack.

    Traditionally reeds were made of leather but since the development of polymers and plastics around the 1950s the leather valves are tending to be left behind.Since leather is a natural material quality control of thickness, stiffness etc. is difficult whereas these qualities are more easily controlled in the manufacture of plastic. It is noted elsewhere that plastic valves could be made from mylar (a product of the DuPont Co.) and the term vinyl has also been noted in their construction. In continental Europe they are referred to as 'vents' or 'ventile'.

   In the largest of the plastic valves it is common to incorporate 'vileda' material (also a man-made material) and as well as providing a good air seal will help to reduce noise when the valve closes and is probably lighter in weight than the plastic. Plastic valve 'stiffness' or rigidity is varied by using different grades of material but also by adding layers of plastic in the style of the older type leaf vehicle suspension spring.
    A valve must re-act quickly to air pressure and while at rest it must lie flat and close to the reed plate. To re-act quickly it must not be too stiff or rigid and must open fully to allow adequate air flow. If it does not open fully this will affect the pitch of the reed, how quickly the reed starts and how it sounds (it may be muffled and have less volume). If it opens too much it follows it will take longer to close resulting in air loss and the slower sound decay of the reed affected. Also a valve opening too much may touch other parts (such as a vibrating reed on a neighbouring reed block).
    A valve should be carefully chosen to match the size of the reed it serves so that it performs in a satisfactory manner.

                             Tired Valves.

    After long use valves become 'tired' and tend to curl upwards and away from the reed plate. It is essential they lie flat and close to the reed plate when idle as even a small amount of air leakage affects the sound and pitch of the reed opposite.
    There are around 250 reed valves in a full size piano accordion treble end and in considering an overhaul (as opposed to only a single faulty reed) it will be necessary to decide whether to renew all the valves or only those that appear to need renewal. Complete overhaul is a large undertaking as it will be required to re-tune all the reeds afterwards and the decision will depend on a few things such as the value of the instrument against the time required etc. One repairer is noted as saying that if more than 50 per cent require attention then all the valves should be renewed.
   Leather valves can be given some life back by 'stroking' them with this simple tool made from a piece of wire (eg. a paper clip)