A mouthpiece assembly includes a base connected with a hose and the base has a base tube in which a spring is received and a seal member is inserted in the base tube. The base tube has two symmetrical outlet notches defined in a top edge thereof and the seal member has two outlet holes which can be in alignment with the outlet notches when the seal member is moved to compress the spring. A mouthpiece is mounted on the base tube and has an outlet which is sealed by the seal piece on the seal member so that when the user squeezes the mouthpiece to move the seal member, the outlet is opened. A cap is removably mounted on the mouthpiece and can be positioned on two lugs on the base when being removed from the mouthpiece.