If I recall correctly, there are O-rings under those washers and they crap out, making the joint leak. The best fix I've found is to replace the O-rings with new ones (metric) and also add a bonded seal washer. I get the bonded washers from Ronald at Ranch Hand Supply. No leaks after that.
There are o-rings under the fittings, check to confirm the size of your fittings (The book does not address the size changes) there are 2- different sizes 14″ mm or 16 mm