Captain reports he noticed a large difference in brake temperatures between the left and right Main Landing Gear (MLG) brakes on an EMB-170 aircraft during preflight. While taxiing out; the left MLG brakes did not respond to pedal pressure. Maintenance re-attached both left MLG brake ratchet type hydraulic connectors and brakes worked normally.

Narrative

During preflight; I noticed a large difference between brake temperatures on both the left set of main landing gear brakes and the right set of main landing gear [brakes]. The left set of brakes were at 8 and 9 degrees C; while the right main brakes were both in the high 280-290 C range. I thought it was strange; but assumed the brakes were used unevenly on the landing roll due to the strong crosswinds on the surface that day. After pushback and taxi out I was testing the brakes and the right brakes worked; while the entire set on the left main gear did not respond to pedal pressure. We arranged a return to the gate via tow; and Maintenance Control was notified.After our return to the gate; we were met by local mechanics from a Contract Company. During their inspection; the mechanics noted no brake puck movement during brake use on the left main gear; while the right main brakes worked normally. Maintenance Control advised this has been a common occurrence; the cause being either the brakes lines were not hooked up properly after scheduled maintenance or the ratchet type brake line attachment to the main gear brakes has a history of backing off of their attachment; rendering the brakes inoperative (there is no safety wire to keep the brake hose attachment from backing away from it's fitting on the main landing gear. It is also not clearly noticeable during a walk around inspection). The brake hoses were re-attached and the brakes worked normally. During the subsequent taxi-out we did not notice any problems with the brakes. Suggest [using] lock-wire or safety wire on the brake hose would keep the hoses from backing away from their fittings.