It's been a long time....

[doctorwayne]

Back to the drawing board ...sorta.

The Delrin stirrup rungs kept "breaking" off....

I'm guessing that "breaking" means that they're not staying put, rather than actually breaking off.
 
LePage has a super glue meant for engineering plastics.  It's a two-part offering, with one being ca, and the other an "activator". 
The activator is similar to a marker pen, and you simply rub it on both of the parts to be joined. 

In your case, it would be possible to apply it only onto the mounting pins on the Delrin steps.  Allow it to dry for about 60 seconds, then apply ca to only one of the parts, and put it in place.
 
I'd suggest putting the ca on the mounting pegs of the steps or, alternatively (and likely neater), installing the prepped and dried steps, then applying the ca to the area where they enter the mounting holes - the ca should be drawn into the joints by capillary action. 
I use the tip of an old #11 X-Acto blade (in its handle) as an applicator...I simply place a small puddle of ca on my glass work surface, then dip the tip into it to pick up a small amount.  Touch the blade to one side of each mounting peg, then to each opposite side of the same peg - this draws the ca completely around the peg, insuring a tighter fit

The ca is in a 2ml "squeeze tube", while the activator is in a 4ml container, and will work with any ca.  I got mine at Canadian Tire, and it seems to do the trick.

I disliked Delrin sill steps for the same reason, although if you can drill holes to create a really good friction fit, they'll stay in place fairly well. 
I'm currently rebuilding a couple of hoppers that I modified almost 20 years ago, following an article in RMC.  I had always thought that they turned out rather well, but re-reading the article and looking more closely at the photos of the real ones, I can see so many places where improvements should be made.
 
For these cars, I used two-rung Delrin sill steps from Tichy, and was fortunate to get a good fit, applying ca after they were in place.  To re-work the two cars, I had to remove some of the added details (one of the cars got dropped, and removed some of its own details, including parts that could have otherwise remained).

Because I was planning to re-work the car's sidesills, the Delrin steps had to be removed, too.  While I could have simply cut them off, I used a pair of smooth-jawed pliers to carefully grasp each one, and without breaking or distorting any of them, all were removed without incident.  

When I re-install them on those two cars (and on the other ten on which I'm currently working), I'll use the prep, as even if your drill bit is exactly the correct size for the mounting pins, chances are the pin vise will not always remain perpendicular to the work-surface as the holes are drilled, usually resulting in a few loose fits.

Here's one of the two originals...

   

   

Wayne