Chrysler Repair: Dodge Caravan 1990 - brake bleeding, vacuum gauge, vacuum line

Question
I had a problem with the left rear wheel cylinder leaking and replaced the cylinder, drum, shoes on both rear wheels. I then replaced the front disks and pads and bled the system using a clear tube in a bottle. With the engine off I have good brake pressure. When I start the engine, the pedal goes to the floor. I have bled the brakes several times now and I get no air from the lines. I also bled the system from left rear to right front (crosswise)as indicated in the manual. No luck.
When I take the vacuum line off the booster, I get pressure back but no stopability. I even tried bleeding the lines with the engine running. The engine idle changes slightly when depressing the brake pedal when the vacuum line is attached and there is good vacuum going to the booster.
the changes in idle seems to indicate the booster is working and the pressure felt on the pedal with the engine off seems to indicate the master cylinder is ok. Any help would be appreciated.  

Answer
Hi Ray,
It seems like the power brake unit is not working properly. The '89 manual I have notes that you may mistake "pedal drops to the floor" when the issue is that the power unit has "run out" (gone past the point of maximum assist). That may be caused by less than optimal vacuum due to leaks in the supply line or by a leaky power unit. The basic test is to step on the brake several times to exhaust the vacuum. Then depress the brake pedal to light pressure (15-25 lbs), start the engine and see what the pedal does. It should drop slightly, hold at that point and require less effort at that point. It isn't clear to me what the next step is if it "drops to the floor" except that you will need a vacuum gauge to do any helpful testing from that point on. I would check the vacuum line and fittings for tightness, including the line that goes to the cruise control. If you have a vacuum gauge, attach it to the cruise control vacuum port on the power brake unit.
See if the vacuum is 12" or more at warm idle.
If you get 12" or more than the power brake unit may be defective. If you get less than 12" recheck the vacuum lines and verify that the engine is idling properly.
Roland