Smokin' gun, I would bet on...
I once had my mechanic do some front work to my 99 Silverado and he somehow left the brake hose with too much slack. The next day my brake pedal went straight to the floor on the first light leaving work, with very little brake power. I was able to stop by a Home Depot and then saw the the line ruptured because the rim was making contact on the line, and abraded it away.
If the hydraulic system has a leak, it can't possibly build pressure (or not enough pressure, depending on severe the leak is.)