I just bled all four brake calipers and my brake pedal is hard when the car is off and pumped up. But, when I start my car up my pedal is spongy. What could be the cause of this. Could it. Be my brake booster or my abs box I talked to a auto part manager he said it could be the abs box. But if it is how can I bleed it there is no bleeder screw on it I need help bad.
Some anti-lock brake hydraulic controllers require the use of a scanner that can access that computer to command it to activate some of the valves. That opens them up and allows air to bleed out. That is only necessary when you allow the master cylinder to run empty. Professionals never do that so they don't get air in the system.
The power booster will not cause a low pedal. It's job is to make the pedal easier to push, that's all.
Why did you need to bleed all four wheels? What symptoms led up to this service?