Ihave posted this before and still use this Valve method with success.
Never leave the dump valves with no liquid in the tank. Any small debrise will harden and cause a leak.
For the black water dump valve I put some mineral oil for storage.
and for the toilet as well.
If you have leaks the system I use is as follows.
1. Flush the system with warm water and then put beach in to kill the boo boos.
2. completely drain the Black Water tank to dry.
3. Pour about a half gallon of water into the tank.
4. Then apply about 8 oz. of pool acid into the tank. leave for about
three days. (watch for ventalation. (fumes)
5 move the dump valve a little to allow some acid to penetrate the seal. and apply more acid. leave for another day.
6 with a pail under the tank outlet there should be no leakage..
7. The toilet I do the same but firstly I open the fluch valve and brush the neoprene seal with an old tooth brush to remove and debrise. Then I put a couple of quarts of water and then about 4-5 oz of acid into the water and leave. you should bea able to see if any liquid leaves away.
REMEMBER ACID INTO WATER...NOT WATER INTO ACID.
This has worked for me for many years.
Do not put tissue into the toilet. I use a little paper bag and never do I get tissue stuck into the valve.
The Thedfor valve system is the pits. I have two repair kits and an extra valve here for any one who has a few bucks.
I live in the Hamilton/Burlington Ontario area.