Here's a weird one: Pedal firm w/o booster connection; drops to the floor if it is!?!

[BanditDave]

Hey Guys,

I finally got my brakes completely bled out on my '78 TA LS1 Swap project.

When I do not have the DSE booster hooked up to the motor and the vacuum line (again from a '99 Camaro LS1) plugged appropriately, I get a perfect pedal and the wheels stop turning. Once I hook up the vacuum line to booster (DSE Master Cylinder Booster Combo) the pedal drops to the floor...ALL THE WAY to the floor!!! Unplug it, cap it off and hard pedal again.

Does anyone have any ideas??? This is new to me...

Thanks,
Dave