Boolean doesn't become true

I have this script, and it makes a car go forward and backwards. When i press w and it goes forward, and i release a car it checks a boolean called slowDown, and it works.

When i press s and it goes backward and release the key it does not check slowDownB Can someone see what i did wrong?

 #pragma strict
 
 var spin = 3;
 var topSpeed = 20;
 var speed : float = 0.0;
 var slowDown = false;
 var topSpeedB = -10;
 var slowDownB = false;
 function Update () 
 {
 //This to the next comment will be the car going forward.
 if(Input.GetButton("FORWARD"))
 {
 
 speed = speed+0.2;
 
 transform.Translate(Vector3(0,0,speed)   * Time.deltaTime);
 
 }
 if (Input.GetButtonUp("FORWARD"))
 {
 slowDown = true;
 }
 
 if(speed > topSpeed)
 
   {
     speed = topSpeed; 
     
     
     }
   
     if(slowDown == true)  
      {
      speed = speed-0.2;
      transform.Translate(Vector3(0,0,speed)   * Time.deltaTime);
      
      if(speed < 0)
      {
      speed = 0;
      
      slowDown = false;
      }
      
        
          }
          
          //This will be the car going backwards.
          
          if(Input.GetButton("BACKWARD"))
          {
          speed = speed-0.2;
          
          transform.Translate(Vector3(0,0,speed)   * Time.deltaTime);
          }
          
          if (Input.GetButtonUp("BACKWARD"))
          {
            slowDownB = true;
          }
          
          
          if(speed < topSpeedB)
          {
          speed = topSpeedB;
          }
          
          if(slowDownB == true)
          {
          speed = speed+0.2;
          
          
           transform.Translate(Vector3(0,0,speed)   * Time.deltaTime);
           
           if(speed > 0)
           speed = 0;
           slowDownB = false;
           
           }
          
        
          
 
     }