My PSU is a 9v 500mA but when I test the output, I find 13.60 volts. Why ?
Exactly. When you test a 9V power supply without load, it is normal that it delivers more than the nominal power. Often, that means that the power supply is not regulated. Normally, when the power supply is wired with one or more pedals, the power should be back to 9V. You can easily verify it with a daisy chain. One output of the power supply on your pedal and the other output to the multimeter. You can verify the value and see that it is back to about 9V.