petrol mower cannot prime rubber perished?

the rubber bung which needs to be pressed 3 or 4 times to prime my petrol mower has perished. Cannot start the mower. Underneath the rubber there are 2 holes, one slightly bigger than the other. How does the rubber prime the mower? Can I inject petrol or air into one of these holes to simulate the action of the rubber? Any other suggestions very welcome. Thanks. The mower says classic 35, cannot see make as worn off.