My truck drying my antifreeze or something

My truck is out of service because me hanic said my motor sending to much
pressure to my radiator..well i turn it on few weeks ago infront of my dad  and he
said my truck sounds fine,so mechanic dosent know what he doing i guess..well
before he even said that he put a thermostat cuz it didnt have one,so new
thermostat,new radiator and new radiator cap,2 new hoses.i pour antifreeze on
truck and water and put a piece of cardboard in the bottom to see if it had any
leaks,next day i did not see no leak on cardboard but did not see any fluid on
radiator next day ,so i turn on truck and 5 minutes later water is flowing up.so
where the coolant go,could it be that a bad water pump..when i turn it on it sounds
like some grinding sound by the fan....and if i drive it the tempeture gauge goes up
hot  but seconds later it cools down and never goes up again..what could it be