Parts Washer Help

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Hey all,
I have a solvent parts washer, the standard drum type (15 gallon drum) with the pump submerged about halfway down the tank. I recently cleaned it and got rid of the old solvent which was no longer any good. I got 5 gallons of Gunk brand solvent from Napa and it barely covers the pump. I don't really want to add more solvent (5 gallons was over $100) but i'm starting to suck air so,
I can probably throw a couple large rocks in the bottom of the tank which would raise the level but have never seen this done....
Or what about adding a couple gallons of water? Would this work? As long as the water level stayed below the pump?
Has anyone done this or heard of it before?

Thanks in advance..