i'm so happy I found this thread.. it finally works! Same problems as yours, I have an upgraded Mk2s to Mk2.5s MMU2S and never changed the heatbreak. I've read about the different heatbreak, but thougt it couldn't be that much of a problem. But after several attempts of changing parameters and trying to change the ramming settings, I quickly realized, that there couldn't be the problems in the ramming settings alone, because i made a testprint with 5 different settings, where every setting was drastically changed to see a difference, but all of them were pretty much the same. The diameter of the filament was too big, always around 2.1mm. So I searched and found this post and thought, let's give it a try.

New Heatbreak and new teflon tube with inner diameter 1.85mm and it runs without a problem, like it never had any problems.

Thank you so much, a lot of people really need to see this.

I had already contacted info@prusa3d.com about my frustrations before I tried the PTFE tube switch. They suggested I was the victim of heat creep due to the thermal pasted needing to be replaced, or that I needed to turn down the temp of the heatbed because excess heat from that was somehow going through the print, the nozzle, and then to the heatbreak, I guess....So many people had success switching out the heatbreak that I decided to try emailing them again and copying the weblink to this thread to see what they had to say. They didn't even mention these findings. It was suggested that I should replace the heatbreak if I WASN'T using the MMU2 with my MK2.5S.

So after not getting what seemed like actual help from them, I everyone's advise from here and I ordered the new heatbreak. I installed it last Friday and BOOM, everything works fine! Thank you everyone who posted on here about your success with this switch.

Some people here commented on poor initial build quality being a possible issue. Before I switched the heatbreak out I had already disassembled and reassembled the thing a couple times. I really doubt build quality was anybody's issue. Also, Prusa did suggest to me that I reapply the thermal paste. Well, I did have to take their advise on that, but only because I took all of the advise from all of you about switching out the heatbreak and new thermal paste is part of the install.

Thanks again everyone, and hopefully many more people can find this thread and solve their issue.

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My MMU2S arrived 2 weeks ago and I got it all assembled early last week. I followed the same upgrade path as most of you MK2S (July 2017) -> MK2.5 (Sept 2018) -> MMU2S w/ MK2.5S (June 2019) so I had the original MK2S heatbreak.

I printed my first multi-material model (the 2-color sheep) last week and it went OK, but 3 or 4 times during the print I encountered a load failure which I had to correct. At some point in those failures it also skipped a layer or two, but overall I was happy with the first attempt. I created a very simple test print with 4 10x10mm cubes each a different color. My goal was to just make minor adjustments to cooling/ramming settings using a roll of Prusament as my "control" since I figured the defaults in Prusa Slicer should work for that. I couldn't get through a single layer without a failure! Even the Prusament would get a very "fat" tip that would jam up when trying to get loaded into the hotend.

Installed my MK3 heat-break from PrintedSolid yesterday and kicked of the test print again. I was prepared to catch failures and adjust from there but it just chugged through the whole test without a single load or unload failure! I kicked off the Sheep print again and a couple hours later had a perfect print without a single failure!

Thank you all, especially @eric-s20 for the suggestion! More testing to come, but I think that fix is a silver bullet for those having trouble with the MK2S -> MMU2S upgrade path.