Black water tank valve

This is a easy fix for valve repair Valve Repair

A few years ago I had that problem. I actually used a rubber hammer on the handle to close the valve but then managed to open it up again. We were 2000 miles from home. I went to an RV place where we were and they said the whole valve had to be replaces but it would take 10 days to get the valve there. I said no, thanks as we would be on our way home by then. I bought an end of line valve, put it on and used that until we got home. Took the 5er to my RV place and they told me it wasn`t the valve. It was the cable itself that was inside a sheath and was binding. They replaced the cable which was extremely long and it is still working. I did have to get the gray water cable replaced with a rod and found out that the valves all face the door side of the trailer and not the side where the push/pull handles are. Makes sense to me....Anyway they managed to twist the gray water valve so it faced the right way and now the rod works good. There is a way to lubricate the cable inside the sheath but I can't remember the name of the part.

From YEARS of experience, there is no way to get a RV dump valve cleaned out without taking it apart. No tool will do that. The knife valve has impacted the debris into the valve cavity. Pushing HARDER will just impact the debris more firmly. Doug

Empty the tank and rinse out. Take a wire coat hanger or a 1/8" welding rod and flatten one end ( hammer and solid surface to pound on). Bend the rod into a 90 degree bend about 1" from the flattened end. With the valve open use the flattened end to clean out the slot that the valve gate moves and seats in. While there lube the slot with a generous amount of water proof grease. The green lube that boat shops use on outdrives and outboard motors when changing props works real well and stays put. I do not remember the name of it tho.
I bet your local boat shop will be familiar with it tho. I have an old tube from years past and it works super and lasts forever.
This suggestion also applies to the grey water valve. While there , might as well do both. By the way ! some rv accessory stores sell a tool for this Made somewhat like what I have described.

Push straight in.. I've had to do that a few times.

Was afraid I would break it

push harder. seriously