I first tried lubing the cable with no success. But I have had good results with mineral oil in the tank. You have to empty the tank first, then close the valve and pour in about 2 pints of mineral oil followed by a little water to flush the oil thru the P-trap. After a few hours, open the valve and drain the oil/water mix into a bucket. Open and close the valve several times to lube both sides of the valve blade and seal. Repeat the process on both gray tanks, then use the mixture on the black tank.
My valves are still pretty hard to open, but only about the first 1/4 inch of movement, then they open very easily and close easily.
Since you are having a problem with the smaller gray valve,if like many of us, you leave the gray valve open, you might also experiment with keeping it closed until time to empty the tank to keep the valve and seal from drying out. Also, while there is no tank flush on the gray tank, you should try flushing the tank whenever possible to clean the tank and valve as much as possible.
If all else fails, drop the underbelly fabric and check the cable for sharp bends or other problems.
As others have stated, an add-on valve on the main outlet is a good backup plan in case a cable valve fails when open.
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Larry Day
Texas Baptist Men-Retiree Builders member since '01
13 Silverado 3500HD D/A, 2wd CCSB srw, custom RKI bed
11 Cruiser CF32MK
https://www.picturetrail.com/dayle1
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