ok sorry for the real crapness of the diagram but its late and Im tired!



Im not 100% sure that the markings are correct on the Amal valve. You might have to experiment! or someone else can confirm? But I think with the switch on the dump valve works. When u switch it off the dump valve will stop working.

If you swap pipes R and W around it will work the other way. I.e. with the switch off the dump valve works, with the switch on the dump valve doesnt work.