I had a 351C pull ATF through the modulator. Was the strangest thing I've ever experienced:
The car was new to me, was slightly overfilled when I checked the oil. I did an immediate oil and filter change, tune up, rebuilt the brakes and put in A-arm bushings (a real chore), and started using it on a daily basis.
Checked the oil and after a week's time, the oil was a little high. I chalked it up to the car not being level when I changed the oil. Also noticed the ATF was a little low; topped it up.
Next time I checked the fluids the same dam thing. Let it go like that for a while longer until I was sure I was seeing the engine gain oil and the tranny lose fluid. Checked the modulator line and it was full of ATF.
But the strange thing was, there was NO visible smoke. Nothing. Nor did the tranny (FMX) shift abnormally in any way. A new modulator fixed the problem. Still puzzled as to how it could draw enough ATF to actually raise the motor oil level, but not smoke like a chimney. It was almost as though it was only doing it when the engine was off, but I know that couldn't be, unless there was some type of siphon action taking place...
This isn't going to help with the problem, I believe the '40 DeSoto was manual-only, but that may be wrong. But a fluid drive didn't use a modulator, either.
BTW, the '40 MOPARs used an electric wiper motor. But the AMC wiper/fuel pump deal is definitely a candidate for "Weird but True".