For those not familiar with changing the knife valves, simply loosen the four screws holding the valve in place. Loosen each screw SLOWLY, one at a time. Do NOT remove the screws completely, just loosen them. As you do so, the valve will start to separate and the "fluid" will begin draining out. Have a pail or similar container under the drain to catch the fluid as it drains out. Empty as necessary, then repeat until tank is empty.
If the tank is empty, simply remove the four screws, separate the valve from the connection and remove. When reinstalling, ensure BOTH large O-rings are securely in their respective housing recesses before tightening.
Finally, as Jeff, found out..if one valve fails, it's a good idea to replace both...they're not that expensive and it'll save you the trouble of removing everything twice. While you have the valves off, run a hose inside the tanks in both directions to ensure clean and trouble free dumping for years to come....Dennis