To check your grouphead flow rate, remove the basket from your portafilter and ensure the dispersion screen and screw are installed. Engage the grouphead at full brew pressure and use a shot glass or other container with a known volume to measure the quantity of water dispensed. You should get around two ounces of water in six to eight seconds. This can vary depending on the temperature set at the brew groups or by the amount of obstruction in the ruby jet due to limescale buildup. If the heat exchangers have any scale buildup, it can also affect your flow rate.