Check the diaphragm on top of the pump and make sure it has 50 psi
Put both the suction and bypass hose into a 5-gallon bucket full of warm soapy water and let it run for roughly 5 minutes. After doing this, run it again with another 5-gallon bucket full of water.
Make sure the lines are clear of debris
No, only nonflammable liquids. 30% solids on 7560 and 35% solids on the 9910