American standard flush valve

Toilets wear out from mineral deposits. Fixn may cost more then new. Googl used commodes first

The overflow pipe should be attached to a small clip that holds it in position over the over flow pipe. If it is loose or attached to the outside it will do what yours is doing.

Check to be sure small tube is flowing inside the ovefow tube. This is the water that flushes the rim holes, and refills the bowl. If you don't have flow clean or replace the float valve.

is the flapper weighted? if it is it will cause it to shut prematurely. one thing you can do is increase the water in the tank. you might have to buy a pvc cuppler to put on the over flow to allow it to fill up more. korky makes a nice flapper thats not weighted

Readjust the float. the longer it is the more leverage the fill water will have on it. good luck. The fill valve does come apart and cleaned if there is debri in it. Turn the water off first:)

hello, this is a standard toilet you will need to inspect the flapper that is the rubber flap that holds the water up in the tank it works when you pull the lever it pulls the chain upward and lets the water flow if this flapper is old it may not stay up long enough for the water to flow. open the tank top and flush the toilet see if this flapper is staying up long enough and giving the tank enough time to empty if not change this rubber flapper its inexpensive part.
if this is not the reason then you need to adjust your water filler valve there should be a small screw on top of the pivoting float arm
if you have to adjust it for the water limit line inside the tank this will be the proper amount of water needed to flush once completely

also check that the chain lenth on the pull lever is not pulling the flapper down quicker than its supposed to,

try this let me know your results
hope it helps

get a new filler assembly. the water in the tank is heavy enough to close the seal completely. or the rubber seal is not making contact. thats' why the leak.
change the whole assemble ususally correct the problem.
adjuste the flow to about 1/4 to 1/3 under the over flow. if that still don't give you the pressure. you ca adjust the flow tube to 1/5 under the cover. that is the max.