I'm rebuilding a rear caliper one a 76 914 using existing parts and the small rebuild kit from PMB and their very helpful YouTube video.
The issue I'm facing is that the clip the lock the spring and seat in the inner tensioner is not staying in place (the clip isn't). I'm able to push the pin in and it stays until in install the piston and I push it in then i can hear the clip pop out and the tensioner doesn't align with the hole in the gear and the piston can't be adjusted properly.
Just spent several hours dealing with it.
There is one obvious damage to the components and I was able to rebuild the rear driver side caliper without much issues. This caliper had a frozen piston that was grinding the brake pad. Piston inner parts appear ok and I had them out and cleaned them before resetting them inside the piston.

Does anyone have any suggestions? Have you experienced this before? Thanks in advance.

Just finished myself. The video is clear, but you might have missed this - you need to use a clamp to compress the inner adjuster spring. This exposes the groove for the clip. Your photo shows that it is not in it's groove.

Mark, the photo was just for the parts. I used the clamp and I can get the clip in the groove but once the piston is installed it pops out.

Make sure the groove where the C-clip goes in is clean. Your caliper doesn't look too clean in the picture.
Check that C-clip us straight and goes in all the way. The one you're using is an EXTERNAL C-clip that's not exactly what ATE intended to have there. It works as long as the groove is clean and the groove lips are not damaged.
If this doesn't help, try a new INTERNAL C-clip.

Thank you all for your replies.
The caliper had already been cleaned and greased back (which is why it appears dirty in the piston chamber).
After removing all parts, turning the clip from a side to another and reinspecting the groove inside, I retried and the clip stayed in this time.

The side in which the clip went In seem to have made a difference.