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That port is the recommended place for no pressure return. That isn't your problem. Problem may be the type of selective control valve you have. The standard valve for 6420 is the 200 series valve and they are not designed to have flow compensation when other functions are used. The optional 300 series valve is what you need. The cap on top of the valve is where you adjust your detents. DOes your valves have 2 position or 3? You need the 3 position valve to operated consistent speed for your vacuum. If you do have 300 series valves, then you may have a sticking valve or broken spring in compensator valve.
 

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The 300 series valve should have three positions to choose from. One has a loader boom picture which has no detent. another picture is a cylinder which should have what I call a kick out detent. You use that setting when you want the detent to hold the lever until cylinder reaches the end of its stroke, then it kicks back to neutral automatically. The third setting is for constant flow such as you want for a hydraulic motor. The knob only controls the detent operation. It has no affect on hydraulic flow. Parts marked "N" in diagram is compensator valve. Remove the plug and oring from bottom of the valve. Then remove the compensator valve. The upper 4 parts of "N" should come out together. The top and bottom parts are threaded into each other. Make sure they are threaded in all the way. They have a history of unscrewing part way. I always screw them all the way apart and clean oil from threads, then apply Loctite before screwing back together. The top part has a smaller diameter land in the middle of it. You can grip it there with thin needle nose vise grips and the bottom part has a screwdriver slot. Tighten it as tight as you can get it and reinstall it.
 

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It does seem unlikely that both would fail that close together, but in mechanical, hydraulic, or electrical systems, anything is possible. The following is a quote from the service manual about the compensator valve:

"The compensator valve maintains a constant flow of oil to an operating SCV. The flow rate is held constant in the range from 6 L/min (1.6 US gal) up to the maximum pump capacity even when other functions (i.e. steering or another SCV) cause the pump or secondary pressure (E) to increase. "
 
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