A couple of hard stops engaging the ABS then bleeding the system seemed to work for me. Thus far at least.
Then in your case, the problem is not related to the abs pump, but to the rear caliper or master cylinder 😯I bled the rear brake circuit on my '16 TV several times. The pedal would be firm for a few months but eventually it would get soft. Even bleeding per the MV service bulletin resulted in the same outcome, over time.
No problems with air in the system, since installing a brake line from the master to the caliper.
The ABS pump is no longer connected to the rear master cylinder or caliper.Then in your case, the problem is not related to the abs pump, but to the rear caliper or master cylinder 😯
I will come back to the ABS unit later, but first I tried bleeding the system again but that did not work for a very long time (I have made it easier by installing banjo bolts with bleed nipple on top of the ABS unit). My MV dealer suggested to install a rear brake master cylinder from a known good bike and guess what, the same problem came back after a few weeks so my attention was back to the ABS unit where, during standstill, on the rear brake side there is a small vacuum at the ABS unit due to the difference in height between the reservoir and ABS unit. For the front brakes the reservoir is above the ABS unit so there is always a small overpressure at the front brake side of the ABS unit and that has no issues. The problems with loss of rear brakes also happens a lot to Ducati's and guess what they also use the Bosch ABS unit installed above the reservoir.My rear brake was gradually losing firmness and after a period of standstill of several weeks, waiting for MV parts (don't ask....), the rear brake was gone and the lever went all the way down. Bleeding the system only worked for a short period and eventually the rear master cylinder was replaced by the dealer under warranty. It was perfect after that but slowly the lever lost a bit bit of firmness and now that the bike is under cover during the winter I checked after a few weeks and guess what........no rear brake. I put the bike on my lift and checked but could not detect any leakage whatsoever so I'm also slowly moving my attention towards the ABS unit as this is the highest point in the system and a leak in this unit might not show as an oil leak as air could enter at the highest point, the ABS unit, without showing as an external oil leak. Haven't had the time to look further. Was thinking about bleeding it again and keeping it pressurised by hanging a weight from the lever and perhaps I can detect a possible leak. Problem of course is that the direction of sealing is different for something going out (oil) and air going in.
So I moved the rear brake reservoir to the tail section of the bike and above the ABS unit. I used brake fluid resistant hose and a small clutch fluid reservoir and made a bracket for this. See below pictures.