Deep well sumersible looses pressure

[drh16]

Hi forum, I installed a new 3/4 hp submersible pump,it is 100 ft down in the casing.When I let the water run at full pressure, after about five minutes the pressure starts to drop then goes to zero.It seems to be running out of water or maybe the well is caving in.Can anybody help me trouble shoot this . The well is 30 years old has a steel casing,I was told it is three hunderd feet deep.The pressure is set at 40/60. Thanks drh16