1, pressure cylinder seal leakage;
2, control valve damage, lead to directional valve can't work normally;
3, reversing valve is too dirty, or its seal damage, cause it can't move;
4, two piston joint damage, split pin fall off, two piston rod, lead to the piston rod can't move;
5, driving the piston seal "wear and tear, cause the piston upper and lower side mixes up.