I've had to work with old rubber also, and it is a pain. I found that there were a few things to be careful of.
Make sure any old adhesive is removed, otherwise you may have a lump underneath the rubber or glass that will cause problems on remounting.
Check for tears, cracks, or gouges. Not only are they potential leak areas, they can be the beginning signs of more problems.
If possible, immerse the whole gasket in a tub of very warm / hot water. This will make it a bit more pliable and easier to insert. If it is still too stiff after this, don't even bother using it, the elastomers in the rubber are long gone, and all you are going to do is tear it trying to get it into the car with a new windshield inserted.