We didn't. Would that cause the problem to happen some time later?

 

There is a hose from the intake manifold to the brake booster. Make sure it is connected at both ends, no holes.
The check valve is the plastic part that the hose connects to on the booster end. It is supposed to allow air flow in one direction only.
Calipers slide, sometimes they stick. If the inner pads were worn much more than the outers is clue.
50% wear on the rears is way worn out. Adjustment is the question here though. The "self-adjusters" are not good enough.

Are there any warning lights or warnings that you are not mentioning?
You should get the OBD read, there could be some codes that do not cause a CEL.

 

Maybe take it to a competent mechanic... Brakes are nothing to play with..

 

Might be an issue with the abs module as well