Working on a 97 2nd generation blazer.
Have replaced Master Cylinder and Booster. Thoroughly bleed brakes. When I hit brakes they go to the floor, after some hard pumping light goes off and brakes will work. Within seconds hit the brakes and the petal goes to the floor and again no brakes. I checked the Porportioning Valve with test light, when brake light is on and no brakes it shows it's engaged. When brakes are working and light is off it's showing disengaged. Is the Porportioning Valve bad? Or is something else causing it to engage and disengage?