If it has not always done this then look for something restricting the suction hose from the input to even inside the pump. If it has always been like this, then different hunt. I am assuming the pump is on your 3pth but will hold true for pulled one also. Couple of things to check first, the tank should have a cut off valve and filter before the pump. Check to be sure you are fully opening the cut off valve and the filter is clean. If you have the hoses pumped direct to the pump "I" would install what is called banjo fittings between the hoses and the pump. They are high quality quick connects for such use.Get the ones with push release pins on the levels. I always installed mine where one hose had male fitting on it and the other hose female. "IF" the hoses are same size it allowed you to connect them together but the biggest reason was to be sure did not reconnect backwards to the pump. Now be 100% sure you had the hoses connect to the correct port on the pump. Does it have a "SUCTION" type hose on the suction side and not a regular hose. Feel the suction hose for a soft or kinked spot. If on the lift check to be sure hoses are not being crimped at the position the lift is use at. With the tractor off and banjo fittings installed and water in sprayer, remove the pressure or out hose on the pump and see if gravity is pushing the water through the pump. If the sprayer tank is above the pump it should. If it is the problem still could be the pump but if flowing good then reconnect the pressure hose and test the sprayer now. If it works fine you might need to do this to help the priming but could be issue with the pump. Do you have a recirculation hose installed? There should be one runs over flow from the relief valve at least back into the tank. If it is not open to work such as setting pressure valve too high or not installed or crimped either the pump can not push the air through to pull the water. (Be sure this is working, it is to keep your chemicals mixed while using. Some settle rather quickly.) Easy to check this, again with just water in the tank and having bleed the water through the pump take the lid off you should hear or see the water being pushed back into the tank. On your size sprayer it might be coming back in the bottom or the top. I really like have the banjo fittings on my pumps. It allows quick bleeding, removing pump to rinse the tank and pump, allows using pump on more than one sprayer, and I remove my pump and drop it in a bucket of oil when not in use. When I began the placing in bucket of oil pump life increased a good bit for me. I use to spray glyphosate, liquid nitrogen and other chemicals with lower price roller pumps and got good life from them. All of my thoughts have been assuming you have the lower price pump, a roller pump. Very good design for many sprayer needs. However there are pump designs that must be primed or you will damage them.
