The first incidence occurred around [XXX]. I was traveling [XXX] on [XXX] in North Carolina about an hour north of Charlotte. I pulled into a rest area and, upon pressing the brake pedal, heard a bit of a hissing noise and suddenly the brake pedal was extremely hard and the car was very difficult to bring to a stop. I did an online search for what might be wrong and discovered it was likely a brake booster failure. I drove the car onto Charlotte, carefully and with extreme difficulty, and took it to the Swedish Connection, a local Volvo specialist. They confirm the diagnosis and replaced the brake booster. Last night, [XXX], I drove the car down to Pickens South Carolina for work. When I got in to return to Charlotte late at night, I turned the ignition, put the car in reverse, and, with my foot on the brake and waiting to back out of the parking space, I heard that hissing noise again. Sure enough, the brake booster had evidently failed again. I drove the car back to Charlotte, mostly on [XXX], and the electronic braking seemed to work OK under cruise control maintaining speed and distance between other cars in traffic. Of course, driving the vehicle on local roads, through stoplights, and turns was extremely difficult and dangerous. I have returned the car to the same mechanic that repaired the last brake booster. I will be reporting this to Volvo as well, as this is a serious safety issue and defect with my vehicle. The mechanic has confirmed that this has happened in other Volvo XC 90s. The vehicle has NOT been inspected by the manufacturer. INFORMATION REDACTED PURSUANT TO THE FREEDOM OF INFORMATION ACT (FOIA), 5 U.S.C. 552(B)(6)