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Jean-Eric Vergne says that the brake-by-wire system is hampering Toro Rosso's progress at the Australian Grand Prix.
Vergne and team-mate Daniil Kvyat completed more laps than any other driver pairing but Vergne was struggling with the new system and had several off-track excursions.
The Frenchman set the 11th quickest time in second practice.
"Sometimes you've got this massive recharging at the rear, which recharges too much, so you have locking at the rear," Vergne told Autosport.
"Sometimes it doesn't want to recharge and the brake goes all forward and you have no brakes on the rear and you lock the front and just go straight in the gravel - so it's quite good that we don't start the season in Monaco!"
Vergne was nonetheless upbeat about Toro Rosso's progress considering his braking problems.
"But performance wise today was not too bad considering the problems I'm having on the braking right now.
"We can work on it. To have succeeded the day how we did with this problem was really good, and I believe if we can get over this problem we'll be in really good shape."
