Cat 740 Brake pump

Hi guys we have a problem with a 740 series 2 (b1p) dump truck. start it cold in the morning no brace pressure you have to rev it up for about 3 mins then you can hear the pump cavitating and then it builds pressure and works fine. basically the pump and suction hose runs dry over night because 1 morning i went to it disconnected the hoses on the pump and wer all dry. i replaced all the hoses to the tank and the o-rings the hose going down through the tank is steel and is not cracked. it has me baffeled now as i taught if anything the pressure side of the tank was at fault it would leak. Any suggestions? :Banghead