the random behaviour is probably due to a faulty or failing pressure sensor which is located inside the pump.Aye it only has to hold up for an hour or so to get your ticket.
I was wondering, I don't know if the ABS unit needs bled via software on these. On vag cars it's a nightmare if you get any air in the ABS unit, you need to get VCDS software and purge the unit.
Just wondering if some air is still sitting in there causing this random behaviour.
I would definitely check for a level sensor too. Should be a plug going into the reservoir which you may be able to get a multimeter on or bridge the connections to diagnose.