Review: When this valve will fix your Delonghi espresso machine.

I bought this part to fix a Delonghi EC580
I suspect it is a common cause of the following symptoms in many malfunctioning Delonghi espresso pump machines.
Symptoms --- Hardly any water leaves the group head when pulling coffee despite the steam wand seeming to put out full pressure and the usual amount of water.

Other (rather obvious) things to consider before replacing this over-pressure valve ---
De-scaling (doesn't help)
Group head blockaage with lime-scale or coffee grounds.

Cause----- is this over-pressure valve (often incorrectly called a safety valve) which sits on top of the pump which is defective (worn-out).
It's a fiddle to fit but will for a bit of time & effort, but minimal cost will restore your Delonghi machine to as new working condition.
Please be sure, before disposing an old valve, that you remove the small black rubber o ring from the old valve (located on a side where you fit in a white plastic tube) and place it in a new one. This will ensure a tight fit.
A YouTube video shows what goes wrong inside the valve and a temporary fix ... Google YouTube Delonghi ec680 problème pression to watch it.
You may also need to buy a tamper proof / security bit set to unscrew the screws (Torx 20 heads with central post to prevent unscrewing with standard T20 bit) in the base of the machine
...a Draper security bit set costs about £4.99
So total cost of repair would be a max of £13.49 ... not bad to restore a very expensive and rather beautiful appliance!!
An essential YouTube video to show you how to dismantle the EC680 or EC685 can be found by Googling YouTube Delonghi EC680M Dedica Water Pump Repair (but you don't need a new pump for the above symptoms)
Some listings for the over-pressure valve refer to it as a safety valve which is erroneous. It is in fact an over pressure valve / pressure regulation device to maintain the perfect brewing pressure to prevent a bitter coffee brew.
If you are interested in a fuller explanation, there is a comprehensive very informative YouTube video found by Googling Espresso Machine Pressure Release Valves.
There is also a very comprehensive pdf with an explanation about the OPV (over-pressure valve) in the better quality coffee machines accessible by Googling Coffeetime Over Pressure Valve (OPV)

I hope this helps someone!