Small four-wheel tractor air brake tappet is made from plastic, and its diameter, lengthoften vulnerable to the effects of changes in air lead to brake failure.

• 小四轮拖拉机的气制动阀挺杆是由塑料制成的，其外径、长度往往易受热胀冷缩的影响而改变，导致气制动阀失灵。