1997 E150, 5.4 engine.
I just replaced the front calipers and used speed bleeders to bleed them.
Brakes feel good until I start the engine. Then the pedal goes to the floor. I disconnected the vacuum line to the brake booster, plugged the line and the intake to the booster. Then the brakes work with the engine running. Can't figure out what is going on.
No sign of fluid leakage around the master cylinder, and the booster check valve passed the blow in both ends test.
BTW: I think this van has 4W ABS. Does this mean that we mere mortals cannot bleed the system?

 

According to my trusty Haynes, that is the case...

 

While those speed bleeders seem to be great ideas (and I have no direct experience with them) they can cause problems retaining brake fluid---those might be your problem here.

There is a procedure for bleeding the ABS module, called a service bleed in most manuals. This can be done by most any competent independent shop or you can spend a few bucks for this sort of device: That would be a "very nice" tool but there are others quite similar in function for a bit less money.

HTH