Help With Brakes

[justgotatruck]

Was driving and there was a sudden burst of air that came through the vents, then the brakes went really weak. I had to push them all the way down before i got pressure and then had to gently work them at that point to not skid the rear tires. Stayed like that for the rest of my drive home. Power steering works just fine, no issues

Any help on what to look for? only thing I've ever done with brakes is change them, so i'm lost as far as where to start. I checked the hose running from the PA and it looks to be in good shape