Hold on a sec Tomas -

The forum sometimes has problems with notification of a thread response and this is one of those instances. First, congrats on solving your problem. Downstream of the priming valve is a tee which can send water to the boiler fill solenoid, a check valve, another tee, and finally the OPV.

I SUSPECT that the check valve (metal ball & spring) has failed, allowing some very hot water to reach the small rubber "mushroom" in the priming valve discharge. So, it might be a good idea to operate the machine sans covers for a while and see if that area of the priming valve gets, er, very warm to the touch. Perhaps it is time to break out the wrenches once again

Thanks Eric,

Just ordered one just in case and as soon as I get it I will replace it.

Is the priming valve necessary and what purpose does it serve? I only ask this because I recently plumbed in my ECM Cellini (same internals as Giotto) vibe pump machine with an ECM kit and the directions stated to remove the priming valve. I have been operating this way for a few months and have not notice any change in operation (but plumbing in has made me happier and had a positive effect on the taste of my shots). Which suggests that the valve may not actually be needed. But I don't know and I have no real knowledge on this.
George

When in operation, the suction line for the vibration pump is full of water and provides the pump with what would be termed a "flooded suction" under all reasonable reservoir level conditions. If these conditions could be maintained (as is the case for a plumbed-in machine) or perhaps, self-generating as in the example of this Bezzera machine, a priming valve would NOT be necessary.

However, the majority of vibe pump machines discussed here have a suction hose run to the top of the reservoir and then led to the bottom of same. If the suction hose became empty for whatever reason, the pump would need to lift its water supply about a foot - a daunting task if the air has nowhere to go - notice I did not say impossible. The air priming valve most definitely facilitates this operation.

A side benefit (?) of the priming valve is that it extends the preinfusion process as it diverts a portion of the pump output back to the reservoir until it automatically shuts at a pressure of 6 to 8 bar.

What really counts is:

. . . plumbing in has made me happier and had a positive effect on the taste of my shots.

Very informative - thanks for the detailed explanation.

. . . plumbing in has made me happier and had a positive effect on the taste of my shots.

Couldn't agree with you more.