With the body cleared of all of its flash, I decided to first make the underframe fit into the body. It is better to do that first when there are no detailing parts attached to either part. The underframe needs to fit inside the body. The next photo is a close-up of one end while I have the underframe held snugly against its correct position at the other end. This gives you an idea of how much material needs to be removed to make the two parts fit length-wise. The instructions say to remove materials from the inside of the body's ends, but I decided to make the underframe shorter instead. I basically filed it to the first board line at both ends. The kit's instructions yield a more accurate model, but it is much harder to do. I am less concerned about what the underbody of the car looks like, so I chose the easier path.
Similarly, the width of the underframe was too large. In the next photo I show by how much. I am holding the underframe against the inside of the body in the top of the photo and you can see how much it sticks out at the other side. Again, here too I removed material from the underframe (you can't really remove material from the body part).
This photos shows the underframe fitting snugly in the body. This took quite a while to accomplish. The way I did this was I clamped a file to my workbench and then filed the underframe part. I did the same number of strokes on each side with about even pressure. That way an attempt is made to remove the same amount of material from each side of the underframe casting so that it remains centered on the body. If you look at the inside of the body, just below its bottom edge, there is a lip on top of which the underframe is to sit. To get the underframe into the body (because there is still a fair amount of pressure on the body, especially in the door area where the part is the weakest), I put one end of the underframe in between the body's sides near its opposite end, and then slowly slid it through the whole body part. That way the sides of the body naturally flare out as the underframe moves through. I then used a thin tool (file or knife blade) to slightly lift it up when the underframe gets near the other end while at the same time pushing the raised end into the body. Note that the underframe is not yet glued to the body; we are just getting it ready for later, so that all this test-fitting doesn't need to be done when all the delicate details are on the body or the underframe.
I set the body aside and started working on the brake details of the underframe. The instructions are fairly clear on this, mostly because of the several photos included. The confusion with the instructions are because the photos don't show the same parts as those recommended to be used in the model. For the main brake line I used the green florist wire that comes with the kit (not shown or referenced in the instructions, but it was the largest-diameter wire included). This main brake line crosses the center sills, so I drilled a hole at an angle at the same location as indicated in the photos in the instruction document. I first drilled all the main line holes with a small drill bit, to pre-drill the holes.
The angled-holes and the end holes in the truck bolsters are easy to do, but the others require you to drill at an angle. In the next photo you can see my attempt at drilling these. By drilling the final hole with a drill bit slightly larger than the florist wire, the fact that you have to drill at an angle still makes the wire fit the hole.
I cut two lengths of the florist wire and routed them through the holes. Near the center, I carefully bent them using a pair of curved needlenose pliers and then pushed their ends into the angled center sill holes. They met in the middle, so I applied superglue to their ends to hold them in position. I then applied a small drop of superglue into each hole where the brake line went through to seal up the hole.
The instructions indicate to install the main brake parts next. I glued the valve to its mounting position. Be sure to mount it such that the five-holes side is facing inward toward the center sill. I found no need to drill out the holes in the valve part. As indicated in the instructions, the reservoir does need to have holes drilled out (there is a small starter-point in the part on each side of the center flange). I used the recommended #76 drill bit. I tried using Testors liquid cement glue to glue these parts, but they didn't hold, so I am using superglue throughout.
I then cut two lengths of phosphor wire and formed them to go between the reservoir and the valve. It probably would have been easier to do the brake cylinder one first, since that one goes under the two shown in the photo. The two from the reservoir go into the top holes of the valve (at least on my version!). Note that in the real world, the reservoir is actually two reservoirs in one, one for regular use and one for emergency use, hence the need for two pipes from it to the valve. I then installed the brake cylinder. The instructions of the kit are a bit confusing about that part, but basically it seems like you need to cut off the brake cylinder rod that sticks out from the Grandt Line part. This I did, as seen in the next photo.
I then formed the wire that represents the piping from the cylinder to the valve. I put that into the center hole of the valve's five holes. I shortened the brake cylinder rod and then glued it to the longer-of-the-two levers that are made by SMMW (the instructions state not to use the Grandt Line ones). Use the second hole on the lever of where the two closest holes are. While the lever was still loose, I cut and glued a very short piece of the chain to the lever (it is easier to lay it down and let the chain lay flat while the glue sets). The lever "floats" and the fact that the brake cylinder rod has been cut off, there is nothing to which to mount the lever. So, I measured, cut, and glued the staple nearest the cylinder at the correct position such that the lever would line up with the brake cylinder. I could then glue the lever to this staple and the brake cylinder, holding it while the glue set. Make sure the lever is horizontally level. This required a few deep breaths and lots of patience! Once that was dry I could glue the second staple in place.
Here is a top-down view showing the piping from the brake cylinder to the valve. The piping could have been a bit more straight, and the brake cylinder rod isn't straight coming out of the cylinder. Oh well.
The shorter-of-the-two levers is glued to the protruding tube on the center sill near the reservoir. Again, this requires a bit of patience and holding it while the glue sets. I then installed the staple to hold the lever more securely. Using two of the clevises provided in the kit and a length of wire, I formed the piping that runs between the two levers, as per the photo in the instructions.
The final piping was accomplished by running brass wires from the levers to the bolsters. The one by the brake cylinder is the one that was connected to the short piece of chain. This one was a bit of a challenge to get it to sit.