Ok, so I got out in the garage tonight and snapped some pictures. Best I can tell the PB booster and master cylinder would actually work fine without the steel filler piece. Not sure why it's required, but I'll get into that in a bit.
Here's the power brake booster (which will be painted black) and the master cylinder pushrod that came with it. As you can tell the pushrod is made of two pieces - a piece of hard plastic and a short metal pushrod that slides into the plastic pushrod.
The pushrod only sticks out a little bit - much different than the stock pushrod that sticks out from the power brake booster by about 1.5" to 2"
I took the pushrod out to look it over and compared it to the stock pushrod. Here's the result.
Here's where it gets a bit weird. The small steel rod from the pushrod goes inside the black plastic pushrod. However, for some reason it's spring loaded. Once inserted the steel pushrod sticks out beyond the end of the plastic pushrod by a small amount, perhaps 1/4" to 3/8". However, if you push on the steel pushrod the spring compresses and the steel pushrod sinks into the black plastic portion of the pushrod assembly. It compresses to the point that the steel pushrod is nearly flush with the end of the plastic piece.
That said, here's the end of the master cylinder that matches into the power brake booster. Note the recess within the plunger. This is what the stock pushrod would typically extend into.
When I bought the master cylinder it came with a steel filler piece that slid into the recess of the master cylinder. It looked like this:
When inserted into the recess of the master cylinder the filler piece filled the entire recess. Therefore, with the filler installed, the steel extension of the master cylinder pushrod would bear directly on the steel filler piece.
So that seems to make sense, right? Install the filler piece into the master cylinder recess and then install the master cylinder on the power brake booster so the steel pushrod actuates the master cylinder.
That all makes a ton of sense except for the fact that the plastic part of the pushrod bears directly on the end of the master cylinder. So, since the steel portion of the pushrod is spring loaded, all the force would be carried by the plastic pushrod even if the filler pieces was installed. What the heck is the point of the steel filler if all the load is taken by the plastic portion of the pushrod? Both the master cylinder and power brake booster are made by the same fabricator so it seems like this must be planned. Gotta admit this one has me scratching my head.
I'm open to any ideas or thoughts on what purpose the spring loaded pushrod serves.