Comme tu dis, libération - Voyez les links:
A compression release engine brake, frequently called a Jake brake or Jacobs brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves in the cylinders after the compression cycle, releasing the compressed air trapped in the cylinders, and slowing the vehicle. - https://en.wikipedia.org/wiki/Compression_release_engine_bra...
Ouvrir les robinets du circuit d'air comprimé du camion {1 par ... tributeur afin de libérer le frein. - https://www.google.pt/?gfe_rd=cr&ei=3vQMWMfBB4bY8gfO8LGYDA&g...

I think neither of your proposals would be correct here; I don''t see any sense of 'libérer', and in fact, 'releasing a brake' means the opposite of 'actionner' — which would be to put the brake on; here, 'release' is about 'letting the brake off'. Were this the parking brake on a vehicle, for example, then 'desserrer' would be the word we would use: 'desserrer le frein à main'. My doubt here, of course, is in knowing in this specialized context what form this 'brake' takes and how this 'manual release' is actually carried out?

Note of course that your text is using 'release' as a noun ('the action of releasing' > 'desserrage'), whereas my suggestion appears as a verb; however, I think the key point here is to first find the appropriate verb for the action involved, and from there, arrive at the relevant noun form required.

ici release of break est mis pour débrayage de frein.