there are some people in facebook group to resolve this problem, Ducati has the exact same problem with rear brake.Looks like a regular complaint. Checked mine to see if it was in the recall group. Nothing there. Its been back to the dealership twice now. 8 hours of my life that I wont get back.
Interesting list. Good info.Several reasons have been identified from what I can see. None seem to be definitive though.
1. Air in the system from not being bled properly to start with.
2. Wrong type of brake fluid.
3. Brake lines too close to the exhaust thereby heating up the brake fluid.
4. ABS module poor design.
5. Bad batch of ABS modules.
6. Incorrect bleeding procedure.
7. Use of hill start causing a blip between the front and rear brake systems.
No.1 referred to the delivery from the manufacturer and No. 6 refers to post rectification bleeding. (sounds a bit like hemorrhoids, has the same effect.)1 and 6 are the same issue, also most report here that the bleeding procedure has been adjusted by Triumph Recall for 2020 Rocket 3 Rear Brake
Had the rear brake problem with mine here in Spain. Dealer in Alicante said it had been fixed on 1st service by replacing the reservoir but I think they only tried bleeding the system. Lasted approx 3 weeks before back to pumping pedal to get it working. Bled system myself and ok for a month or so. Had 1st years service in November and been perfect ever since. They said they had replaced part of the abs under warranty but their English and my Spanish not good enough to tell you which part I'm afraid. I think the morale here is to keep taking it back to the dealers until they get it right because it's a fantastic bike when it's working properly.