Mutchy-1

Have just rebuilt a 3 door Sierra for my boss and having an issue with the brakes, I believe it's a fault with the wiring going to The 2 pin sensor on the brake master cylinder. When ignition on abs pump primes etc as it should and cuts off. But if you press brake pedal the fluid level rises in the reservoir and the brake pedal goes solid. Anyone come across this before and possible things to check thanks

David