I am going to guess the flapper.
My toilet keeps kicking on about every 15 or 20 min. I cant see water running in the toilet from the reservoir, so I put a llittle food coloring in the tank and sure enough the color went into the bowl. Sunday I put a quarter turn supply valve on and also took the float valve out to twist and turn the height of the water level as it was a little low to start with. I did not have problems with it before I did that so I am wondering if it is the flush valve or the flapper? Also if I remember right there is a new flush valve out that will not continue to fill up the reservoir.It seems to me I saw it ina a magazine or on the internet.Anyway thanks for your opinions. Got to get off of here now and head to work.
I'd bet money on the problem being the flapper. Since you get the dye into the bowl, that's the only way it can get there.