Haas has yet to determine whether it will continue to use new brake disc supplier Carbon Industrie during the Russian Grand Prix, after making the switch from previous supplier Brembo this weekend.
The American outfit has struggled with its brakes since its entry into Formula 1 last year, with Romain Grosjean particularly sensitive to the material used.
Grosjean acknowledged Brembo's improvements, but was keen to try the Carbon Industrie discs in order to draw a comparison, and ran the company's products during in-season testing in Bahrain, and again in Friday's pair of practice sessions at Sochi.
However, Grosjean described the day as "difficult" and pinpointed his brakes as an area for improvement.
"We’ve got very little grip," he said. "We’re really struggling with the balance. We had some issues, as well, with the brakes over the long runs. We need to look at what we can do better with them.
"Generally, it’s just been a very difficult Friday. The car didn’t perform well – very low grip on low fuel and high fuel."
Team principal Guenther Steiner admitted the issues Haas encountered with the brakes was likely down to inexperience with the new supplier and a decision on which brakes they will use would be made on Saturday.
"I would say the issues with the brakes were mainly because they’re new to us," added Steiner. "We need to find out how they work.
"Going through the data, we will decide tomorrow what we’re doing and how we continue. All in all, we had pretty fruitful sessions. We did a lot of laps and we learned a lot. Now we need to get the best out of what we learned fortomorrow to go into qualifying."
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