I can see several things you can do, but honestly, simply pushing on that plunger is probably the easiest. Just be careful and not push on the spring loaded plunger to far. One you could break the spring, secondly you can get the plunger cockeyed and it won't seal until you un-cockeye it!
As I said earlier, I use the compressed air method and during that process I have to remember to run the water pump to expel any water inside the filter and pump chamber.
Good Luck, Be Safe and Above All, Don't Forget To Have Fun
2006 Fleetwood Discovery 35H, 2014 Honda CR-V, M&G Engineering Braking System