Help what's wrong with my water tank?

[lee1234abcd]

My small water tank is overflowing and dripping out of the overflow pipe in the alley outside. I'm not very good with plumbing problems but I looked inside the tank and it was full of sludge, which i thought would be the problem. I emptied the tank and allowed the ball cock to fill it with clean water. It was working fine but just went to see tank and it I'd half way over overflow pipe, much higher than before. Why is there more water coming in than being pumped out? I have a metal armed ball cock. Not sure if it needs replacing or how to do it? Any help please.

Lee

 

[seco services]

Check to see if the tank is filling from the ball valve.
If it is hold the arm with the ball on up does it stop filling ?
If not the diaphragms faulty if it does the balls probably not adjusted or faulty.

 

[lee1234abcd]

Yeh it does fill up from the valve and when I pull it up it stops but occasionally it drips a bit, do I need to replace the ball cock and arm, how easy are they to fit