It looks like a standard seal. There will be a metal surround, around the edge, moulded over, buried within the seal material. Not much good applying force to the flexible part, you need to somehow apply it to the outer metal ring of the seal somehow. That's why I mention a slide hammer - the tip, hooks under a lip, on the end of a rod, then you slide a weight up, until it hits a stop.
Not suggesting you buy one, but this is how they work...
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