La Marzocco Linea Mini - Brew gauge at 0 bar, flow delays at group

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Posts: 27
Joined: 8 years ago

#1: Post by ntsimmonds »

Apologies if this has been covered, but I've had a hard time searching the LMLM thread for exactly what I'm experiencing (and gooogling hasn't been any more helpful, either). I have also reached out to LM home, but I figured with the wealth of knowledge and experience here, someone else might know the solution or be able to aid in troubleshooting.

I've had my LMLM since summer 2016 and it's been great. No significant issues, and I haven't had to replace any of the internal regular maintenance parts to date (which, I think, could be reason for what I am experiencing in this post) - only the group head consumables (gasket/screen/screw). I have done no mods or upgrades except reducing pump pressure to 7bar and replacing the brew gicleur with the .6mm one, otherwise all stock from 2016.

I believe both of my issues are related - first, my brew gauge pressure is always at 0bar unless I have activated the paddle and the pump has built pressure and water is flowing. This leads into my second issue - there is a significant (multiple seconds, sometimes 10+) delay between activating the paddle and water flowing from the group, more than the 1s factory "pre-infusion." Generally, the longer intervals between paddle activations, the longer the flow delay will be up to about 10s+, and even a few minute interval (i.e. brewing back to back shots) incurs a shorter flow delay that is > the 1s PI. I'll throw a video at the bottom of the post that demonstrates the issues.
  • This has been happening since probably 2019-ish, but has not impacted actual espresso brewing - once I get pressure at the group, coffee brews just fine and is tasty - so I haven't bothered to address it until now.
  • I am thinking this could be related to either the expansion valve or the 3-way valve as neither have been replaced or maintained. However, there is no abnormal leakage from either of those parts into the drip tray.
  • I tried adjusting the expansion valve both with the machine off and on with a backflush disk in the portafilter, both with no result or change at the brew gauge.
  • Steam wand and hot water valve are both fine.
  • Almost exclusively (99%+) used TWW Espresso packs + distilled water through this machine. When I replaced the gicleur last night, I noticed no appreciable scale build up in any of the parts I removed (I went the "back way" via removing the 3-way).
  • I do leave my machine on 24/7, not sure if that matters but I think less thermal cycles should lead to less wear part replacement (as has been my experience so far)
I am trying to avoid just replacing the 3-way ($200~) and expansion valve ($50~) together because I want to be able to to solve the issue if it happens again. Not necessarily worried about the cost, the machine has been relatively low-cost maintenance wise for this many years, it's about time I incurred something :lol:

TIA for any input, and if there is any other relevant detail I missed please let me know!

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#2: Post by kitt »

If nothing is leaking out of the 3 way or expansion valve, and your brew boiler constantly shows 0 pressure, most likely a check valve (one way valve) is leaking. There are 2 of them back by the pump assembly. I'm surprised you aren't seeing the brew boiler over heating as it looks like it is almost draining. I'd recommend getting fixed soon before you damage other components

ntsimmonds (original poster)
Posts: 27
Joined: 8 years ago

#3: Post by ntsimmonds (original poster) »

kitt wrote:If nothing is leaking out of the 3 way or expansion valve, and your brew boiler constantly shows 0 pressure, most likely a check valve (one way valve) is leaking. There are 2 of them back by the pump assembly.
Thanks for mentioning this kitt - based of your comment, I literally went on a 2-day deep dive into the LMLM manual and other check-valve threads (primarily La Marzocco GS3 Brew Pressure Always Zero When Idle on a user's GS3 from 2015) that corroborate the idea. It makes sense, too, that if the particular 4-way cross that the LMLM Brew Gauge branches off is not able to hold pressure, then there would be either a leak or a backflow issue. I have no leaks, so the next problem to solve is potential backflows. The machine is 7+ years old, so I don't think it's out of this world to assume multiple checks have busted o-rings/springs. Apparently, LM released a bulletin a few years back that these were upgraded and went from around $35/piece to $70 :cry:

I watched tons of videos, stared at the manual, and looked at internal pics of the machine to decipher the water path in my head and then just made this diagram (not the cleanest...but better than nothing :lol: )

So what I am seeing, and based on the manual, there are 4 total check valves (same as the GS3 ones) from the pump forward. I put big red arrows pointing to them, boxed them, and then numbered them in sequence from the pump.

I am guessing replacing #1 (pump outlet) is for sure going to re-pressurize the system toward the brew boiler, but there is still a possibility that downstream check valves are busted. Then #2 is the check valve for the cold water stream into the brew boiler, and #3 is the check valve for cold into the HX - both of these have to be busted if the machine is not pressurizing at the brew gauge, so I am 100% planning on replacing those. And #4 is the check for the cold stream into the steam boiler, and I imagine if this one was busted, the 2-way solenoid ("A") could also possibly act as it's own check (maybe?). And whenever it opens for a fill, the pump at 9bar is > steam pressure at 1.5bar, and so would not I am not particularly concerned about #4 but might go ahead and replace it anyway.
I'm surprised you aren't seeing the brew boiler over heating as it looks like it is almost draining. I'd recommend getting fixed soon before you damage other components
I actually think I am, but not hot enough to pop the tstat (no idea how, and unfortunately I snapped the button off whenever I was replacing the gicleur, so yet another part to replace). Since the issue started years back, I noticed that the top exterior panel over the grouphead would get hotter than I remembered, but I chalked it up to maybe just misremembering. Since I typically leave the machine on 24/7, over the years, the white paint has actually started to discolor a little bit from the rest of the panels, so there is definitely something going on there. I have the machine off for now while I wait for parts.

In any case, thanks again, and I'll be updating this thread once I am finished just for posterity - a thread from 2015 helped me, so hopefully this helps someone down the line.

For the record, I still haven't heard back from LM Home (not sure what their turn time is on emails), but I am pretty certain that the check valves are the culprit here.

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Joined: 17 years ago

#4: Post by kitt »

Hope you get it sorted, prices seem typical LM prices.... not sure why $70 in USA when ; ... a-treated/

ntsimmonds (original poster)
Posts: 27
Joined: 8 years ago

#5: Post by ntsimmonds (original poster) »

Greetings with an update!

I went ahead and swapped all 4 check valves just so I could be starting from fresh and not have to go back in and replace the steam boiler one too soon. My hunch and research was right - every single check valve was bad in the tank -> grouphead lines. Of course, except the one that splits off after the T to a 2-way solenoid to open the water line to fill the steam boiler. I suspect that one doesn't see as much regular flow/use as the grouphead, so less open/close cycles. Even though I replaced it, I did keep the original in case I need to swap one out and I don't want to shell out the money for one more.

This job actually only took about 15-20 minutes with the proper tools once I got the case off (itself a 15-20 minute job lol), so if you are referencing this post and you are worried about how difficult this job could be - it is easy!

You will want to have a 16, 17, 18, and 19 mm set of crescent wrenches/spanners (or two 8" adjustable wrenches would work, they're just bulkier). All these check valves are attached to very fragile copper lines so it is absolutely paramount to have good positioning of your arms/body relative to the machine whilst holding the wrenches to break the connections (I had my machine on my dining table, and I stood facing the open side with the plumbing, perpendicular to the machine), and have one wrench on the check valve and one on the connecting nut being broken. I nearly bent one SET of lines because I was being careless.

Once all the old check valves were removed, I replaced the on the pump first as it was tucked back the farthest, AND it had teflon tape on the old one, which I thought was odd, but I reapplied 4~ turns of heavy duty teflon tape to both connections on the new valve anyway. Once that was done, I got all the new check valves connected on the bottom/ground side to their connections, and was able to just place the multiple T-split mess right back in place and land on the top sides of all the valves. Ezpz lemon squeazy, now the group holds pressure and I have no flow delays when activating the paddle!


Kitt - I just realized the link you posted was for a check valve at $30 USD which is insane. I have no clue why they are so much more expensive here and if I had realized earlier I would have ordered from that site - I did manage to find them at $52~ USD, which is better than $70 but still kind of ridiculous. That link should prove helpful to the next one who needs the new check valves, thank you for sharing it!


I also took the opportunity to rebuild my steam wand valve and that proved to be a much harder task that the check valves and took two sessions as I didn't have proper tools the first time. I had to buy a bench vise, clip ring pliers, a small hand awl, and a 2nd adjustable wrench to get the job done. This video from LM on YouTube shows the exact process, which I followed, for Linea steam valve rebuilds - they are identical to what is on the Mini, but the Mini uses a shorter wand.

After this, I thought that maybe that $700 cool touch wand might be worth it :lol: but that's another purchase for another holiday.