I know I've got the black cap - remember it from when the transmission was out to R&R the rear main seal. I'll check it for cracks.

The hose is good - I checked that at the same time.

I'm assuming a quickie test would be to use a mity-vac on the engine side and see if I can pull any vaccuum at all. I don't know if the modulator would ordinarily "hold" a vaccuum, but if the cap is cracked, I'd think I'd be unable to pull any vaccuum at all.

Thanks for the suggestions.