1997 900s. the low coolant light comes on. I check it, it is low. i unscrew the cap to relieve pressure and the reservoir fills up past the full line. put the cap back on and drive it, the light comes on again. i check it, and, again, it is low but fills back up when the cap is taken off the reservoir. how can i fill it if the reservoir keeps filling up? is this a sign of something else wrong? please help!!!!!!!!!!!!!!!!!!!! this is driving me nuts