Noticed a small leak. Cycled the valve and it got worse. Turned out the gasket had rolled. The valve was the original in a 73 Safari, which means it had the metal plate. All the repair kits are for the "all plastic" version, so as far as I know this valve can't be repaired (the gasket is glued into the plate).
Opened up the belly pan. Holy Crap! The valve can't be removed without cutting both ABS pipes.
You can see the metal plate and the rolled gasket in this photo. This valve required the long handle modification, so the hollow corner rivets had to be drilled out in order to recover the handle push rod.
With the pipes cut, it was a piece of cake to replace the valve with the plastic version. Replacing the standard push rod with the long version required totally disassembling the new valve. The main problem with the plastic valve is that it is about 1/2" thicker than the metal version. The Thetford plumbing stack was already proud of the belly pan, so this extra thickness really made the attachment fitting stick well below the belly pan.
Without going into too much detail, as I began reattaching the 1-1/2" ABS pipe, it "came loose." I've never seen a pipe like this totally fail. The break was not the pipe, but a 45 degree elbow right at the point under the bed that the pipe went through the floor. (Just another delightful day of turning a half-day job into a two-day job.)
Now it was obvious that the belly skin was not going to be able to conform to the new, lower face of the Thetford fitting. Luckily this is the Burning Man Airstream, so function is a lot more important than looks. My little brake is only 18" and a new cover was going to be 23 x 9". It could have been fabricated using a plate and some "L" extrusions, but I elected to use a scrap piece of 0.025" skin, some 2x4's, and a hammer (an old homebuilt airplane trick for making wing ribs). It is not nearly as slick as it looks in the photo, but it works.
All back together, with a few new rivets and "washers" to hold up the old belly pan.