The Australian finished fifth in FP1 but encountered the issue in the closing minutes after pulling to the side of the circuit at Les Combes.
Piastri was able to get moving again and slowly return the car to the garage, allowing McLaren to begin investigating the problem immediately rather than recovering it from the circuit after the session.
The team identified a hydraulic leak and was required to complete a gearbox change before Piastri could return to the track in FP2.
Those repairs continued beyond the beginning of the second session, leaving the 25-year-old in the garage while the rest of the field began its running.
Piastri eventually emerged 19 minutes into FP2, shortly after the session had been stopped to clear gravel deposited on the circuit by Max Verstappen at Stavelot.
Despite losing almost a third of the second session, Piastri completed 11 laps and finished sixth with a best time of 1m46.926s.
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That left him 0.982s behind pace setter Kimi Antonelli and 0.792s away from teammate Lando Norris, who finished second after completing an uninterrupted qualifying simulation.
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“Overall, it has been a reasonable day,” Piastri said.
“Unfortunately, we were delayed going out in FP2 as the team worked to fix the cause of the hydraulic leak that appeared at the end of FP1. Thank you to them for working through to get that sorted.
“However, we made good progress in FP2, and our pace is looking reasonable. We are heading in the right direction, and we are in a good place to build on that in FP3 tomorrow and into Qualifying.”
McLaren technical director of applied engineering Neil Houldey said the issue had cost Piastri valuable track time but praised the team for completing the necessary repairs during the session.
“The hydraulics issue that presented itself at the end of FP1 on Oscar’s car was disruptive, unfortunately costing him valuable track time in FP2 as the team continued to work into the second session after swapping the gearbox,” Houldey said.
“It was a strong effort from the team to get through the workload as quickly as possible, so thank you to them.”
McLaren used both sessions on the Friday to evaluate a new lower-downforce rear wing designed for the demands of Spa-Francorchamps.
The component was initially fitted only to Piastri’s car in FP1, allowing the team to compare it against the alternative specification used by Norris.
McLaren installed the new wing on Norris’ car between sessions, leaving both drivers on the same specification for FP2.
Houldey said the component had performed as expected, although McLaren still had work to complete overnight to refine its energy deployment and ensure there were no further reliability concerns.
“The low downforce rear wing we brought has performed as we hoped, which is another positive to take into the weekend,” he said.
“Our main focus overnight will be on fine-tuning the energy deployment strategy and ensuring reliability so that we are able to maximise our track time.
“We’ve gathered a lot of data from various options tested today, but we know the best is still out there for us to find. While Oscar’s lost time makes things a little harder, we are confident in his ability to get up to speed.”
Norris completed 18 laps in FP2 and finished 0.190s behind Antonelli, giving McLaren encouragement over its single-lap pace heading into Saturday.
The reigning world champion will, however, carry a 10-place grid penalty into Sunday’s race after McLaren introduced a fourth power electronics unit on his car.
Piastri remains free of a penalty and will look to make up for his reduced Friday running when final practice begins at 12.30pm local time on Saturday (8.30pm AEST), ahead of qualifying later in the day.
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