Lewis yesterday has had some rear brake issue and in addition to Jerez issues they have here, according to AMuS, a revised break-by-wire - BBW - :
"... The Silver Arrows came with a completely revised electronic braking system. The brake-by-wire had made in Jerez big trouble and the pilots surprised again and again with unexpected brake balance shifts. The surgery was successful. Hamilton reported on the first test day of good progress in this area. The new system gives the rider more confidence. However, an error crept. The rear calipers have not fully opened after braking. This sanded the pads on the discs. When the rear brakes were too hot, Mercedes broke the day of testing."