I just looked up the Marathon slicks and it read they were only good for 50-70. Another similar Marathon was good for 50-85. That sounds more like what I was riding the Slicks at. Normally tires that cap out at say 65-70 are not built to take the higher pressure, if they are road sized. They are probably comfort bike tires, and they just keep inflating like a balloon the more air you add.
Just checked some NOS Slicks I have, they are marked min 55, max 95. But this site says 45-70.
http://www.adventurecycling.org/stor...s-341-tire.cfm
So it probably pays to check the tire you are actually riding.