Why wont my keurig make a full cup of coffee?

Why wont my keurig make a full cup of coffee?

hi … i have a keurig k40 …that every couple months decides to not make a full cup of coffee … tried everything shown in info i seen on this site…last couple times i had this problem …on the top theres a check valve that i took apart ..cleaned etc and watever i actually really did im not sure ..but works than for a month or 2 same issue …would it help changing check valve ? or would this k40 have a solenoid that could be causing issue ?