Valve cores seldom give trouble, but do not last indefinitely. Dirt under the seating will cause a puzzling 'slow-puncture'.
Check that they are not leaking by applying spittle to the end of the valve and watching for air bubbles.
The valve core is of the same type as that used with tubed tyres, and screws into the valve body. The core can be removed with
a small slotted tool which is normally incorporated in plunger type pressure gauges. Some valve dust caps incorporate a projection for removing
If the core is proved to be sound, make a similar check to ensure that air is not leaking from the valve body or from its
sealing face. If this is found to be the case, the valve must be renewed, although it is worth checking that the early threaded-type valve's
retaining nut (where fitted) is securely fastened.
To renew a valve, remove the wheel from the machine (Section 2 or
Section 3) and take off the tyre (Section 11). On the early
threaded-type valve unscrew the locking nut to release the valve and ensure that it is securely fastened on reassembly. The later car-type
rubber-bodied valve (fitted to all models after mid-1986) must be removed by cutting off its inner retaining shoulder and pulling the remains
out of the rim. To fit a new valve, lubricate it thoroughly and push it through the rim from the outside towards the centre (having first
removed its cap). It must then be drawn into place by screwing the correct tool on to its threaded end to provide purchase (the tool should be
available at any tyre-fitting establishment). If great care is taken to avoid crushing the valve end it is possible to screw a suitably-sized
nut on to the valve threads and to grip this with a large pair of pliers. Check that the rubber locating shoulders lock securely into place on
each side of the rim. Refit the tyre and wheel.
A valve cap is a safety device, and should always be fitted. Apart from keeping dirt out of the valve, it provides a second
seal in case of valve failure, and may prevent an accident resulting from sudden deflation.