i changed the thermostat and bleed the water system, gauge worked correct when i left home and as soon as i can in to stopped traffic the gauge started to go back down slowly then the car started to steam under the bonnet!!!

i pulled over straight away and turned off and water and steam was coming out off the water tank! the cap was tight when i left so not sure how it was coming out?

i let i cool down and put some warm water in to fill it back up and got home!! help me please