Washing: Use Nikwax/McNett/Gore Gore-Tex detergent. Rinse a couple of times after washing. This removes traces of soap from breatheable fabric; excess soap left on the membrane will clog the pores of the material. Hang dry.
When 100% dry, spray with Durable Water Repellant available from above mfrs. This allows water to bead-up and roll off the coat, and also allows membrane to breathe. Without DWR, the coat will have a film of water on its surface, and membrane will not breathe. This is the RIGHT way to do it.
Gen I ECWCS coats had a known issue with the sewn-through seams leaking, particularly with, but not limited to, the seams alongside the zippers. Solution for this is to wash the coat, dry it, and then apply some SEAM-Sealer on the sewn-through stitching, inside and out. ONLY THEN apply DRW spray. Some sewn-through stitching will be covered inside by mfr-applied taping; you can ignore these stitched areas. Seam sealer available from camping goods vendors, such as REI, EMS, and Campmor. Water-based is probably the best for this application, as it is almost invisible when dry.
If you do as I have suggessted, you will be GTG.