McLaren team principal Andreas Seidl has warned the team faces a "race against time" to rectify the brake issues that have plagued the second F1 pre-season test.
Lando Norris completed only 79 laps across the first three sessions at the Bahrain International Circuit with the MCL36 spending a lot of the time in the garage with the front-end being attended to.
The British driver hinted at the end of the first day that any fix would not be quick, with the team's concerns no doubt heightened by the brake fire experienced by Williams on the morning of day two.
With just a week to go before the first round of the season, the optimism carried from the first test in Spain is slowly waning for the Woking-based team.
"Well, after the good test in Barcelona, these first one-and-a-half days haven't gone to plan," explained Seidl.
"We have had, unexpectedly, an issue on the front actually with the brakes which we have to solve.
"It is a race against time at the moment in order to get upgraded parts here."
Suggesting the running so far has not all been negative, Seidl added: "We have still made steps with modifications we made here at the track.
"That has allowed us to work on the car and work on the understanding of the car as well, putting some small upgrades on the car to extract more performance from the package.
"But I am optimistic that despite the running time we have lost and the limited laps we could do that, in the end, we leave the track on Saturday well prepared for the race weekend."