You said the bypass valve. Do you have just one? If so then you set the valve so water flows into the water heater, DO NOT TURN ON HEATER AT THIS TIME, leave the drain plug out of the heater (in fact insert a short piece of half inch pipe to route water out past the siding) if it flows, then that valve is working, shut off water.
Now, on single valve systems at the top (HOT WATER OUTLET) is a check valve, It's toast, you will need to either remove it if possible, or gut it if not (Reach in with needle nose pliars and pull the guts out) You can then add a 2nd check valve shown above which will be easier to replace when the need arises.
Second: Sucking pink stuff into the lines is the WORST way to wintereize.. blow 'em out with compressed air,, Open all valves including low point drains, let it drain, then blow with air regulated to 50 PSI, till you either run out of air, or run out of water.. Wait for compressor to recharge, or a few minutes, do it again, like 5=10 times.. Forget the pink, and the taste of pink, Just use PINK in drains and toilets where flushing it out come spring is no tribble at all.