Anyone ever performed this deceptively tricky procedure?

Can’t seem to get the pedal inner ‘trapped’, ie not rotating fully, within its outer cage, having put the new end cap on.

Is there some special knack to it or some blindingly obvious thing Inshould be doing?

Can’t find any tech docs online, only an exploded parts diagram.

Not much help to you, but the ones that i’ve done in the dim and distant past were just a bit fiddly. Sounds as if you’ve not got the coil spring’s legs located correctly if the pedal’s able to rotate.