Do I need a new brake booster?

[elgyqc]

I am preparing my 2000 green base model Boxster for sale, after having completed an engine swap. I went for a test drive and the first time I tried a serious stop I almost sailed through a red light. The brakes work but there is no power boost. I got it stopped but only with after applying a lot of pressure on the pedal.
Today I did the following troubleshooting.
1) pulled the vacuum line for the brake booster off the intake manifold. Engine almost stalled, put it back on, the engine returned to normal.
2) pulled the vacuum line off at the booster... engine almost stalled, put it back on, the engine returned to normal.
So I have vacuum at the booster...
3) In my Boxster S I put my foot on the brake pedal and started the car, the brake pedal went down noticeably, as I would expect with power brakes.
4) Back in the problem car I put my foot on the brake pedal and started the car, the brake pedal barely reacted.
5) Went for a short test drive and the problem is still there.
The brakes worked last fall before I garaged the car for the winter, although it did seem that the brakes were not as good as in my S. During the engine swap I didn't touch the hydraulics. The brake pedal is rock solid, so no air in the system.
I don't see that it could be anything other than the booster.
Anyone have other ideas... I wasn't counting on another $500 expense right now. :-(