Sounds like it can be an ABS issue, does the pedal hop and make noise when braking?- false abs activation- Is the pedal travel normal? Does it get squishy or does it feel hard when pressing? Check for vacuum leaks. That would cause the pedal to get hard, brake booster may have a leak. Is there signs of fluid loss? Loss of fluid can point to a leaky master cylinder. Separate master cylinder away from booster to see if fluid leaking into booster. Another thought can be the abs motor. Sometimes you have to do an abs bleed service procedure with a scan tool.
That's about the best I can do without actually putting hands or eyes on the vehicle.