B737-800 flight crew reported a hydraulic line leaking while the aircraft was at the gate.

Narrative

On day 0 at approximately XA:30Z; Aircraft X experienced a mechanical failure of Hydraulic System B while parked at gate in ZZZZ airport.While conducting flight deck pre-flight duties and with about 60% of passengers on board; the ground crew urgently signaled me (the Captain) to open the flight deck window. He informed me that there was smoke and pressurized hydraulic fluid spraying out of the main landing gear wheelwell. I asked him if he called the fire department and he said yes; that they were on their way. I commanded an emergency deplaning with the Flight Attendants and Gate Agent.I promptly turned off all electric and engine driven hydraulic pumps and the leak stopped. Hydraulic System B pressure dropped from approximately 95 to 84% very quickly with negative secondary indications on the flight deck. I reviewed the QRH with the First Officer for applicable procedures. I decided the non-normal was contained. I pulled the CVR circuit breaker.Simultaneously; the Flight Attendants (FA) commenced emergency deplaning procedures with PA announcements in Spanish and English. I made a PA and instructed passengers to deplane immediately. The number 1 FA informed me that the cabin was all clear. I walked the cabin with the Aviation Maintenance Logbook (AML) and verified that the cabin and lavatories were free of personnel and passengers. There were no injuries. 737 aircraft was deemed out of service by maintenance.Maintenance corrective action recorded in AML; replaced the upper flexible hose leaking on the right MLG and leak checks were good after repair.During First Officer's pre-flight on day 1; residual hydraulic fluid from the leak was observed all over the #3 and #4 MLG tires and strut assembly. The previous day's maintenance crew used water in attempt to clean the fluid. Current day's crew were using Aviation T-Force non-flammable degreaser. I informed Dispatch and maintenance control that this needed to be cleaned IAW (in accordance with) with the SOP. Maintenance control informed me that soapy water is the recommended method. Maintenance complied and I visibly confirmed the effectiveness by observing rust-colored fluid on the ground.After allowing the tires to dry; both the #3 and #4 appeared to have a large splatter pattern embedded into the tires. The mechanic said that this was grease. The First Officer confirmed that during the pre-flight the day before; there was not a splatter pattern on the tires.Maintenance control directed that #3 MLG be changed. During the maintenance; it was determined that the #4 MLG also needed to be changed.After a 5-hour delay; the crew returned to a hotel for crew rest. A few hours later; Aircraft X returned to service. The third pre-flight yielded an airworthy aircraft and the flight crew landed in ZZZ safely. Wear and tear of hydraulic components exposed to adverse weather environment and under high pressure may have caused the hydraulic line to rupture.Once the Captain commanded emergency deplaning of the aircraft out of the abundance of caution; the Flight Attendants' procedures and training to direct passenger movement towards the open L1 door and jet bridge was very efficient.Ensure flight crews verify the maintenance corrective actions are in compliance with SOPs and maintenance control direction. Flight crews should carefully manage and assess the risk during the course of a non-normal; response and corrective actions to ensure safety of ground operations and flight.