If Russia paid enough, people would steal the brakes. They might even steal them themselves, and report as stolen. Then they get more, and who's going to investigate some brake pads if the owner isn't pushing for action on the case?

There absolutely is a black market for selling airplane parts to sanctioned countries. As an example, Cuban state airline Cubana has been flying ATR turboprops with American made engines (and many other parts) since 2002 or so. Even with wide sanctions, Iran has maintained an operational fleet of old Boeings, and during past decade even newer aircraft.

https://www.aerotime.aero/articles/31395-here-is-how-iran-sm...

There is a significant amount of goods moving via Georgia. To be exact they move them from Georgia proper to part occupied by Russian separatists. That is totally different from smuggling stuff to Russia. Exports from Germany to Georgia were something like 50% up year to year.

How strict is that manufacturers' control over spare parts?

A few days ago there was an article about European merchants evading the trading bans on a large scale. They'll get the brakes. If Russia can afford their price.