i know there have been posts about this before but I am going crazy with these prest valves,.. have a serfas floor pump w/ gauge

when i try to check the tires the pump complelty deflates the tires i kee trying to get air back in but the pump just keeps poping off then when i try to get it on again it deflates. i know which way toput the lever on and the guy at the bike shop was able to us it but these valves are complelty hyper sensitive it's like one touch and they complelety deflate agian.. is there such a thing as setting them too loose. i feel like i have to hold the pump ont h valve to get it to work but then i can only pump with one hand

advice please ! i am getting so frustrated!!!