petrol stimmer

I have a petrol strimmer I am not sure what make as all names have fallen off but it looks like a mc cullock, just cleaned carb but when reassembling I managed to damage the little rubber bung inside the primer bulb, I think it might be a non-return valve of sorts,anyone know where to get one from, most strimmers seem to have this,but don't know where to get one, or indeed what it is called,anyone any ideas
cheers