B737 Captain reported experiencing disagreement and resistance from other departments after refusing an aircraft for an uncorrected hydraulic system leak.

Narrative

Aircraft X. ZZZ-ZZZ1. During preflight walk around; an excessive amount of hydraulic fluid was observed in the right main wheel well and on underside of fuselage of Aircraft. Maintenance came out and did a System A and B pressure test moving flight controls and Flaps down then up. They couldn't find any leak and signed off the logbook. I looked in the logbook and the day prior Hydraulic System A had been written up in ZZZ2 for the quantity dropping to 58%. The system was serviced and signed off. The fluid on the plane and the logbook write up a day earlier we're just one big red flag to me. I've never seen a system that low from 'normal operation.' That and the fact the wheel well was covered in fluid were my evidence for my decision. This is when I told maintenance I was refusing the airplane because it was obvious to me Hydraulic system A had a leak. My first refusal notice to ops and dispatch was around D-10 to D0. About 10 mins later I received an ACARS message from dispatch: 'maintenance wants you to show them the leak.' I then called maintenance control and they were basically saying that no leak was found in the test and the aircraft was signed as airworthy and were questioning my refusal. I explained the fluid and logbook history. But right about that time Line Maintenance came back to the cockpit and we had another discussion. Their argument was that no leak was found with the test; but my argument was all the physical evidence: The fluid coating the inside of the right wheel well and the fuselage along with the previous fluid loss logbook history; which I couldn't ignore. They also kept saying that our current system A level was about 81% and there is no way to know what the level was a day earlier after the service in ZZZ2; indicating: how do you know if it's leaking if you don't know what it was compared to now? I kept reiterating that the servicing has nothing to do with it. It's the fact that the system level dropped to 58% on its own which is way below dispatchable level. Even if the plane departed ZZZ for ZZZ2 the day prior with the absolute minimum of 76%; it would have lost 20% in one 6 hour flight. This and the wheel well coated with fluid were the big indicators that there was likely a leak of some kind. I continued to refuse the airplane. At approximately D+20 I called the FODO (Flight Operations Duty Officer). He had already been briefed by dispatch. I explained in detail the situation and the reasoning of why I was refusing and that maintenance was telling me the airplane was airworthy. He was in complete agreement I shouldn't take the airplane and stood by my refusal decision. The FODO called me back about 15 mins later after making some phone calls and he advised me they were finally getting us a new plane. This was at D+35. We went to the new airplane; loaded up; and departed about 2 hr 7 mins late. I already submitted a report and I mistakenly didn't select the X option. So I'm trying to keep the two the same as best I can more has come to mind since. Basically I stand by my decision to refuse because it was the obvious safe thing to do. I was feeling very uncomfortable about the airplane and honestly whole situation. I stood by my decision for putting safety as the first priority.Lessons learned: Arm-chair quarterbacking this whole thing days later; I now see that I should have involved the FODO at my first refusal notice. I waited way too long to call him. But there's a reason for that; and it's my previous experience. I can only remember maybe one or two instances in the last 18 years where I had to refuse an airplane. In those cases; it was refuse and 'here's your new gate and airplane Captain.' No questions asked. That is what I was expecting to happen when I refused this aircraft. I sat back and waited for things to happen because that's what my previous experience told me in that situation. But instead I received non stop push back and pressure to take an airplane that had a blatantly obvious defect. My call to the FODO is what seemed to finally get the different departments to finally listen to the fact the PIC of this aircraft is refusing and a swap is now necessary. I'm now resetting my knowledge and experience bank for the future. The FODO was an amazing resource and I will definitely use the FODO in a more proactive way in future situations. And for the record I'm not blaming Maintenance Control or ZZZ Line Maintenance. There was an obvious disconnect in communication where they were only concerned about clearing the logbook entry. The line maintenance tech was very professional and definitely was getting a push from above. And maintenance control can't be faulted because they are evaluating what they are given; and they aren't seeing what I'm seeing. The only comment I want to make is I was very surprised the situation went as long as it did. As the Captain and PIC; if I say 'I refuse this aircraft' and there is justifiable and obvious evidence to support this; then there should be no push back whatsoever from any department. We wasted 45 mins with going back and forth trying to convince me to take the plane. In the end they gave us the new plane which was one gate over. It was sitting there the whole time while they trying to reverse my refusal. Operationally; I was disappointed in the process that took place. ZZZ is not an outstation where the plane is grounded and will cancel the flight if we don't take it. It happened at a hub; with maintenance and spare plane possibilities. The safety culture didn't shine the way I expected it to. The only ones saying 'safety timeout' was us; the flight crew. I stood firm and I'm proud of that decision. Update: the next day I received an update email from the ZZZ chief. Maintenance worked on the plane and did a more extensive test. They found a 'very large leak' on Hydraulic System A just above the Frangible Fitting. This happened when they moved the landing gear handle up. The leak test showed no leak at the gate because they couldn't test the gear. In the end it was very good experience for me and my brand new FO. This was his first leg off IOE! He did an outstanding job. He discovered the fluid; wrote it up; and called maintenance before I arrived at D-45. His previous experience as crew chief in military really and Other Company Check Pilot shined. I really appreciate all the positive support we've received from the Flight Ops department. The ZZZ1 chief pilot reached out to us with a pat on the back as well as the ZZZ chief supporting and giving us a thumbs up for making the absolute right call.