If there is no snap ring, I assume you have no pushrod and rod retainer washer fitted. The snap ring holds these in place. The plunger should push in against a spring. If it doesn't then it's stuck, due, probably, to corrosion. A sharp bang down onto a block of wood should pop it out if the corrosion isn't to bad. If no luck you'll have to move on to your favorite free up spray, PB Blaster, Kroil etc and/or apply a bit of gentle heat with a hot air gun. The harder it is to get out the more likely it is that the bore will be too damaged for a successful rebuild.