How do i replace group head gasket on Nuova simonelli mac 2000

is it leaking on the side of the portafilter or what you mean is the dose is short? if dose is short, you can calibrate by pressing the fifth button on that group about 5 sec. then it will blink. prepare you grind in the portafilter then put it in the group. brew with the appropriate button the manually stop at the right level or quantity in the cup. you can do this with all the doses you require. Now by pressing the fifth button again 5 sec. you can exit programming mode.

you can drain your machine by removing the plugging screw in the drain spout. You can see this screw where the drain water all go. it is a black plastic square thing where your drain hose is connected. After removing the plug, you have to open the bypass valve beside it and open all steam valve to make air come in. make sure the machine is cold.

Press and hold the stop button then press the desired button you want to program at the same time to enter programming mode then press stop to desired quantity

It sounds like the metering jet on the top of the group is blocked.

Replace the pump head and or the capacitor. Check the capacitor on the pump motor first before replacing the pump head.

Remove the shower screen and diffuser, this first step is a precaution, then using a scratch awl or an ice pick pry off old gasket, make sure there are no pieces left in the head and clean, then insert new gasket.

The restriction is at the top of the group in the metering jet, most likely the screen is blocked, depending on model your screen and jet is either on the square brass box on top of the head or the small nut on top of the head. DO NOT REMOVE THE NUT UNLESS YOU HAVE ANOTHER GASKET TO REPLACE THE OLD ONE p/n 02280032. Clean screen or replace along with gasket.

The metering jet on the top of the boiler is blocked.

Pressure pump is not working or can not produce 9 bar pressure.
Or the coffee is less tha 7 gr.
Or the coffee is grinded too thick.
Please remebr to vote for this answer thanks

Either the brew solenoid is bad or the shower screen is blocked or the metering jet on the top of the head is clogged.