91 E350 brake problem

I bought a 22 passenger airport shuttle bus w/ a 460 mtr. I drove it from Pa. To Dallas, Texas non stop and it runs strong. It has a compressor that comes on and levels the load. But on my way home the brakes became very soft and took a very long time to stop.I had to really pay attention comming through the mountains. The was the warning light came on and it says 'REAR ANTILOCK'. This is a 30 foot bus and I am taking all the seats out to make an RV out of it. Where would I take a vehicle this large to get it looked at and or repaired? A regular brake shop? Or Truck repair shop? It does not have air brakes and has two axels in the rear one has dual wheels on each side and then a single wheel on each side behind the duals. Any suggestions on where to go? And what the problem might be? And last. How expensive might this become? LOL Thanks Charles sammoon@aol. Com ?