Actually, it’s possibly something to do with the flight model issue, where the flaps are generating too much lift and therefore unweighting your wheels from the runway.
Maybe try raising the flaps straight after you’ve landed, to see if the brakes work any better.
Unfortunately, the Longitude is one of the payware aircraft, so isn’t user patchable. If this is the issue, you’ll need to wait for the next update, Sim Update 3.
Edit: Maybe give the modded CJ4 a go. It’s a blast to fly.