主题：大墙国竟然没有人能懂的该如何做好一杯咖啡实在是大悲哀。 -- forger

I put two youtube video to explain a little bit about the espresso machine internal structure .

https://www.youtube.com/watch?v=2jn28YTS3OQ

It is much more complicated than your thought.

There are three valves to control the pressure release .

The under-pressure and over-pressure will take out liquid with unbalanced emulsion.

Another video explained a little more on temperature and pressure control.

https://www.youtube.com/watch?v=WqBSfikYp7U

E61 HX requires flushing to cool the brew boiler tube aka heat exchanger until its stabilised to the correct temperature and also the exhaust valves are for depressurizing and diluting to prevent the lower exhaust from clogging ?

The middle valve is actually a pre infusion valve. There is not two exhaust valves. When the pump is activated, pressure will push the middle valve down slightly (but NOT the bottom valve) opening up that middle chamber. The filling of this chamber creates a 'delay' in the buildup of the pressure. A soft 'pre infusion' or ramp up

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sorry for English , Chinese input isn't available in office computer .