172_driver
"Not 787 rated but I guess the design of the gear lock pins is pretty universal. Do they depressurize the hydraulics in the retract lines or do they physically prevent the down lock from moving (with hyd. pressure on)?"
Downlock pins (as opposed to steering lockout pins) typically don't interact with the hydraulics and simply provide (indirectly) a mechanical resistance to gear retraction.
Here's an example (actually a MLG, but the same principle applies to nose gears):
The gear isn't quite fully deployed in this shot - when it is, the two highlighted beams pivot so that they are in line with each other and form a geometric lock. You can just see the two holes that will then line up and if needed, accommodate a ground lock pin.
When I first encountered gear pins many years ago, I wondered how a relatively small pin could resist the force of a massive retraction jack - but of course they don't have to, all they need to do is to maintain the geometry of that downlock brace.
See post #22 for the pin in situ.