I'm in the process of doing a similar conversion on my old L1 - Mostly collecting needed parts for now. Instead of starting a new thread, I thought I'd simply add on / contribute to this one.
As a start point, I put a gauge on the group head via the 18x1.5mm plug on top of the group and didn't get the values I expected.
The boiler pressure is set to 1.5 bar but the group gauge only shows 1 bar.
Also, when making a coffee the group gauge remains at 1 bar.
From diagrams, I thought there was a check ball in the group and that gauge would read high(7 bar-ish?) during extraction but it remained flat at 1 bar. I even tested the gauge to make sure it was working properly.
Am I misunderstanding the pressure at the 18x1.5mm fitting?
1. check that your gauge reads zero when the machine is cold. if the anti vac valve on your machine has stuck at some point since 2016 a vacuum is created as the machine cools and this has enough force to cause the needle on the pressure gauge to turn on the centre pin as the vacuum tries to pull the needle below the stop. this then results in a gauge that reads greater than zero when the machine is off and cold, and overstates the actual pressure when the boiler is pressurised by the value of the overstatement when the system is cold. this will be true even if the anti-vac valve was subsequently cleaned or changed
2. you only put a check valve in the dipper configurations of this group. if you put a check valve in with a thermosiphon configuration it is likely to result in the thermosiphon running cold as the machine ages/scale forms in the non return valve
3. to get an accurate pressure reading you will have to bleed out every last cc of air between the water and your pressure gauge or the air compresses under load and acts as a shock absorber and soaks up the increase in water pressure, so you won't see it on the gauge unless you get all the air out
I checked the gauge and sure enough, It reads 0.1 bar high when the machine is cold.
I can hear the anti-vac pop up when the machine is warming up but I'll probably pull it out and clean it to be sure.
I also re-checked the group head gauge I was using and it's not accurate below 4 bar. Cheap gauge :(
Grabbed another gauge, bled all the water out of the fittings and everything is agreeing.
The lack of a check valve explains the pressure spike when releasing the lever mentioned earlier as the pressure will feed back into the boiler until the piston seal passes over the cylinder water holes. A quick release of the lever can bump the needle.
Question, without a check valve and the back feed of some water from the piston to the boiler (or at least into the copper tubes) : Is there any possibility of coffee getting back to the boiler?