I have a 2002 Toyota Sequoia SR5 4wd 260k miles. All new brake check valve , brake booster , master cylinder, 3 separate complete ABS modules with pump and skid control ecu assemblies (used) . Multiple brake fluid flushes . After pressing the brake pedal multiple times the brake systems pressure builds up and all 4 calipers lock up and the pedal becomes rock hard hence why the new booster and master cylinder. I am at my wits end . If you had to guess , what would it be ? Toyota service dept. tried to charge me 2500 for a new skid control ecu and said this would be the next step. This would have have been my last step more like a leap . But I feel like it’s something more simple and inexpensive.  I’m a backyard mechanic that has hit a dead end . Please help ! 

p.s both VSC lights are on , those are the only lights on and not blinking or beeping.