Too much water during preinfusion after replacing E61 mushroom in Expobar Brewtus

Equipment doesn't work? Troubleshooting? If you're handy, members can help.

#1: Post by dunja »

Hi all,

yesterday we replaced the grouphead mushroom on our Expobar Brewtus IV (because the old one was too old and slightly chipped) as well as a few other parts in the grouphead.
After assembling everything together again, the preinfusion failed to work properly: it lets too much water (even though the pump isn't turning on). I remember reading once that someone had a similar question and it turned out they assembled a specific part in an upside-down way, but I'm not sure which part that could be.

The only issue I've noticed is that the o-ring, which came with the mushroom is tad shy of filling the entire slot on the mushroom. As you can see from the photo, there is something like 1mm between the ring and the end of that slot for it. I'm not sure if this is at all a problem, or completely unrelated to the above issue. (the o-ring should be 2.62 mm thick, unless they've sent me a wrong one).

Any help would be very much appreciated!



#2: Post by JRising »

The o-ring looks fine in the photo. They're all a little loose out in the open like that, it will get smashed in position and fill that groove a little more.

But, do take it out and look at it again and look at it's cavity in the head, feel the wall where it should seal, down around the brew-valve's seat. If water is getting past that o-ring and getting under the mushroom, then it is bypassing the gicleur and giving you more water through the head than just what fits through the gicleur.

dunja (original poster)

#3: Post by dunja (original poster) »

Thanks a lot for the reply! I'll definitely do that. In the meantime, I've made a video to show how the flow looks like. Perhaps this is how preinfusion is supposed to look like, it's just that our machine was not functioning properly before, I am really not sure. ... qzz5qz.mp4

As you can see, the preinfusion releases as much water as the flow with the pump. In addition, when the pump is on, the flow makes a kind of stop & go impression, is that normal?

Thanks a lot for the help!

UPDATE: Something else happened a bit later: the machine wouldn't let any water out for about a minute, in spite of the pump working. It looked as if the water just wasn't at all in the grouphead, until it finally reached it.

Supporter ♡

#4: Post by Nunas »

It does not work the way you're demonstrating it. In order for the e61's inbuilt preinfusion to work, there has to be a properly prepared coffee dose in the machine. With no portafilter and coffee, all you're seeing is what's called water debit. The middle valve on the e61 is controlled by a spring that allows the valve to operate at about 4 bar. With no coffee load on the group, that valve never gets a chance to operate.

dunja (original poster)

#5: Post by dunja (original poster) »

Thanks a lot for the reply, @Nunas! This explains a lot the issue with preinfusion (or why there might be none :) )
What about the run of the water which looks a bit intermittent, is that normal?

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Randy G.

#6: Post by Randy G. »

For more info on how the E-61 works, consult the Vibiemme Domobar Double owners manual, (PDF download) pages 5 through 8. (DISCLAIMER: I created the manual but the download is free, hosted on Stefano's website,
* 21st Anniversary 2000-2021 *

dunja (original poster)

#7: Post by dunja (original poster) »

Oh wow, thanks a lot, @Randy. I had no idea such a detailed manual existed, this is really helpful.

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Randy G.

#8: Post by Randy G. » replying to dunja »


Here is an interesting bit of science (I hope). Take note of #17 on the pages documenting how the E-61 group works where I mention that when there is equilibrium in water pressure between the infusion chamber and the brewing area, the force of the spring closes the infusion valve. That information was relayed (in Italian) to Vibiemme (the holder of the E61 patent) and they mentioned they had never thought of that, and that it is likely true.

Also, the benefit of having a plumbed E-61 machine is that when the lever is lifted just enough to open the brew valve, but not so far as to energize the pump, the infusion continues at line pressure for as long as the user desires.

Finally, the patent document shows that there is a "needle valve" in the very top of the group that has been (IMO) mistaken as an adjustable flow device, but I believe that it would not have been used for that. First, consider that the top of the group is very hot in normal use. Combine that with the fact that the patent does not show that this part is threaded. It is shown being held open by a coil spring. To use it to manually control flow during infusion or brewing, it would have to be held down. An uncomfortable thought when it would likely be at a temperature over 140 or 150 degrees F. I have hypothesized that it was to clear obstructions (such as scale particles) from the gicleur. This based on the original patent which may have changed later.
* 21st Anniversary 2000-2021 *

dunja (original poster)

#9: Post by dunja (original poster) »


Concerning the "needle valve" you mention, what would that be in this cutaway view on p. 8? And what's your take on the flow-control kit for E61s?

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Randy G.

#10: Post by Randy G. »

dunja wrote:Interesting! Concerning the "needle valve" you mention, what would that be in this cutaway view on p. 8?

No. That photo in the manual is an actual cut open modern E-61 group. Pretty cool, no? That image was supplied to me for the manual.

This is a clip of the poster I created which clossly duplicates the original patent. I basically traced the entire drawing in Photoshop and I added an image of a spring instead of the line drawing of one in the patent. Note that in the patent they do not show the threads of the assembled body parts of the roaster.
You can view the original patent HERE
And what's your take on the flow-control kit for E61s?
I don't have one other than an opinion. I no longer have an E-61 machine as I now have a Decent. When I shopped for a new machine I was in a position to be able to purchase just about anything. Having only manual control over the flow rate seemed counterintuitive as it would be difficult to repeat consistently, and the thought of being forced to stand there and do so did not appeal to me. If I was to go that route I would have gotten a Londinum or Cremina or other such lever machine.

..but we diverge from the topic..
* 21st Anniversary 2000-2021 *