I had a bad toilet that started running last year. This was in a bathroom that is not used at all (guest bedroom). So, i just closed the valve to that toilet and decided to look into it later. I haven't had a chance to fix that yet but last night but master bath toilet started running. Does this indicate a root cause problem. The house is around 20 years old and may be these toilets are really old too and if so, it is fine. I just want to see if there might be any other reason such as water pressure or something like that causing it. I measured my water pressure on an outside faucet and it seems to be ok, around 70 psi or less. I don't exactly remember that number right now but I remember it was well within the recommended psi.
Run enough water to get the hot water heater to start its cycle, then close everything and measure the pressure until the heater gets to the end of its cycle. If you have a check valve and no expansion tank, this can cause a pressure issue that wouldn't be detected with a quick test. Instead, it will slowly rise as the water heater runs, and then drop back to normal when the first faucet is run.
It's also possible that all the toilets have the same parts and are therefore failing at the same time. But before replacing everything, it's worth it to make sure they are really faulty.
If you dont mind me asking.... Couldn't you just check the flapper? Fluid master Just seams kinda silly to do a pressure check because the water in the tank dropped, check to see if water in running down the inside of toilet bowl. You will always have pressure in the lines unless your toilet trickles out water in the tank when flushed.