As you can see in my other thread. Just did some brake work. The problem now is when the engine is running the brake pedal falls to the floor. It has nice firm pressure when the engine is off.

So I pulled the abs fuse. Same problem.
Re-bled the brakes. Same problem.
Pulled the vacuum booster line off, plugged up the manifold hole. Problem is gone.

So the theory would seem to be that with the new fluid (yes DOT 3) the vacuum booster supplies enough pressure to the master cylinder that the fluid flows from the high pressure side into the res.

Right?

 

The vacuum booster only assists to depress the m/c plunger. The reason it's very stiff when there is no vacuum is because you have no help applying pressure to the system. It's very difficult to stop a car when the vacuum booster isn't operating. (Very stiff pedal)
With that said, it does sound like the m/c isn't producing pressure. Are you certain all the components are working correctly and all the air is indeed out of the system?

 

Sounds like air in the master cylinder, sometimes they need to be bench bled before attaching them to the car.

 

Either air is in the line or the MC is toast

 

when you bleed your system start here:
1. Right Rear
2. Left rear
3. Right Front
4. Left Front
Ensure you keep plenty of fluid in the reservoir...I find the push pedal>open bleed screw>close bleed screw>release pedal.... method moves fluid faster....

 

+1 to above suggestion. Even better method, get a small hand operated vacuum pump (about $30 at Harbor Freight) and bleed from the caliper bleed valve. Same order of wheels as above, vacuum until you have clear fluid coming out, just make sure the reservoir NEVER gets empty. You can do this by yourself, and it's pretty quick. Good luck.

 

For $30 I could get a pressure bleeder. Which I might do.