1992 Mercedes 300SL ABS/Brake problem

Question

1992 Mercedes 300SL.Which has been sitting for a few months.No ABS light on.Plenty of fluid and pressure coming from the master cylinder to the ABS unit/pump (Bosch part #0 265 200 043).If I back way off on the (ABS unit) fittings to the front wheels,I don't get any fluid.I've tried pumping it with the engine off and running.Local mechanics that I talk to seem to think that there is no way the ABS unit/pump would cause this.This unit also has a integrated Proportioning valve.Any ideas ? Thanks!