Push button toilet keeps running instead of filling cistern?

The flush valve isn't closing, allowing the water to run straight out into the pan. Either the button is jammed, or the mechanism is jammed/broken or the large sealing washer on the bottom of the flush valve plunger is stiff or damaged/defective. You need to investigate all of the flush valve and the flush button linkage in order to find the exact cause, or you could just replace the whole thing with a quality one, they are not expensive. I like fluidmaster fill and flush valves, I fit those for customers I care about - however other quality brands are available. There is also a great deal of cr*p available in the WC valve marketplace. I dislike Wirquin, for example.

Sparks (the Plumber)
May 2018

take the top off and see if push down button is sticking on the push down , staying open, take a good look with the lid off to see the problem,

n
March 2018