Why Isn't This Sprint Working?

Why isnt this working? I can see NO syntax problems or incorrectly typed variable names? It just doesnt work, theres no errors or anything, it just doesnt work?

 var MoveSpeed = float;
 var RunSpeed : float = 4.0;
 var RunSpeed : float = 4.0;
 var SprintSpeed : float = 8.0;
 var JumpHeight : float = 6.0;
 
 var Gravity : float = 20.0;
 
 private var MoveDirection : Vector3 = Vector3.zero;
 
 private var ButtonA : boolean = false;
 private var ButtonD : boolean = false;
 private var ButtonS : boolean = false;
 
 
 
 function Start()
 {
     MoveSpeed = RunSpeed;
 }
 
 
 
 function Update()
 {
     var controller : CharacterController = GetComponent(CharacterController);
     
     if(Input.GetButton("A"))
     {
         ButtonA = true;
     }
     else
     {
         ButtonA = false;
     }
     
     if(Input.GetButton("D"))
     {
         ButtonD = true;
     }
     else
     {
         ButtonD = false;
     }
     
     if(Input.GetButton("S"))
     {
         ButtonS = true;
     }
     else
     {
         ButtonS = false;
     }
     
     
     
     if (controller.isGrounded)
     {
         MoveDirection = Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
         MoveDirection = transform.TransformDirection(MoveDirection);
         MoveDirection *= MoveSpeed;
             
         if(Input.GetButtonDown("Sprint") && !ButtonA && !ButtonD && ButtonS)
         {
         MoveSpeed = RunSpeed * SprintSpeed;
         }
             
         if(Input.GetButtonUp("Sprint"))
         {
             MoveSpeed = RunSpeed;
         }
         
         if (Input.GetButton ("Jump"))
         {
             MoveDirection.y = JumpHeight;
         }
     }
     MoveDirection.y -= Gravity * Time.deltaTime;
 
     controller.Move(MoveDirection * Time.deltaTime);
 }
 
 
 
 function Sprint()
 {
     MoveSpeed = SprintSpeed;
 }
 
 
 
 function EndSprint()
 {
     MoveSpeed = RunSpeed;
 }