Narrative:

I went to the cockpit while first officer (first officer) did the preflight. Upon completing preflight first officer brought to my attention a possible fluid leak top of left medium large transport (main landing gear). I inspected the defect and noted fluid present on what I have come to know as 'banjo fitting' associated with brake lines on upper left medium large transport strut. First officer pointed out stains on left medium large transport tires likely associated with wind catching the leaking fluid and landing on tire.I contacted maintenance control who requested the previously mentioned pictures. I also entered a write-up in aml (aircraft maintenance logbook). Maintenance advised non-[company] on call maintenance would be called. Approximately 30 minutes later; [maintenance] arrived and confirmed the fluid present. After consulting with maintenance control; [they] advised me that I would be required to run the left engine for 3 minutes. It is my understanding [maintenance] removed the fluid from the fitting I complied and determined that in feather I could get required pressure. Upon completion of the run; [maintenance] came to [the] cockpit and declared there was no further evidence of the leak. I mentioned to [maintenance] that I wasn't sure we accomplished anything as we never applied brakes or ebrake. He left the cockpit for a few minutes and returned stating maintenance control instructed us to run engine 3 minutes and actuate the brakes and ebrake a few times. At that point I called maintenance control and told them I would need to push back to do the engine run. Maintenance control stated that was not necessary. I acknowledged their comments believing that was unsafe to do at the gate. I discussed with first officer and determined if we captured with the tug and in feather; we could do the test at the gate. My belief was the test done in that configuration mimicked a typical push back operation. I coordinated with ramp and proceeded to start number one and apply the brakes. At the same time; I took the opportunity to migrate brake fluid; 50% #1 75% #2; thus accomplishing the required maintenance request. I quickly realized with ebrake pressure being depleted #2 would need to be started. That was accomplished and maintenance request was completed. With engines in feather I was able to get adequate hydraulic pressure to accomplish the maintenance request. Had we needed to come out of feather we would have pushed back and accomplished on a taxiway.[maintenance] did a general visual inspection and determined no further leaks. It is worthy to note no actual maintenance procedure occurred other than cleaning the fitting and running the engine. Aml was signed off and we departed for sea.upon arrival in ZZZ; first officer left the aircraft to get some breakfast. I did the required post flight and upon inspecting the above mentioned 'banjo fitting' I observed a yellowish fluid evident on the connector. The amount was 2 drips vs the 3 drips observed on the original aml entry. I called maintenance control and made another entry into the aml. At that point I left the aircraft for about 15 minutes to go get a coffee. I returned to find a maintenance technician holding the book and as I recall in the vicinity of the left landing gear. I inquired of the status and he indicated the aml entry was signed off with gvi (general visual inspection) no leaks noted. Thinking that was odd; [I] proceeded to observe that [the] connector was in fact clean and free of leaks. I mentioned that someone must have wiped it off. The technician told no one had. At that point knowing the connector had a leak that reoccurred I told the technician I would not be able to accept the aircraft. As I recall his response was something along the lines of ok. I called maintenance control and explained the situation and they advised me the aircraft was signed off and good to go. I explained to maintenance control I was going to refuse the aircraft. Maintenance control advised me I would need to contact the fodo (flight operations duty officer). I called dispatch and he sent a request to fodo to call me. Shortly thereafter fodo contacted me and I explained the situation. He indicated he would contact [maintenance operations] and maintenance control. [Fodo] called me back and relayed a request from maintenance control that I fly it just one more leg and they would change out my airplane. I declined as I believed there was a leak that required corrective action before another flight. [Fodo] indicated he understood my position and would attempt to secure a different aircraft to finish my day. [Then]; I was advised I had a different aircraft; aircraft Y. Shortly after that I believe the crew started packing up to go to aircraft Y on spot 3. Sometime before the first officer departed to aircraft Y a different technician along with the original technician that I had interacted with came on board the aircraft. He asked me to point out the leak that I had written up. I proceeded to the left engine and pointed out the connector that was at this point clean and dry. He proceeded to tell me it was clean and no leak was noted. Additionally he pointed out these leaks; presumably ones that I had written up; were on all the airplanes and are very common and that's just how those connectors are. He told me that he had not wiped that connector off. Which was interesting as I didn't say or infer he did. He proceeded to then tell me the other side was the same way and 'why didn't you write that one up.' I began to get a feeling this might escalate into an undesirable exchange so I told that technician 'thank you for your observation' and began to collect my belongings for the plane change. After that exchange I observed the two technicians take the aml and enter the walkway out of earshot and examine the logbook and appeared to be making an additional entry into the aml. Before leaving to aircraft Y the aml was replaced being the captain seat. Curious what the additional entry might have been; the last entry was the sign off from my original writeup. I got the distinct feeling that the sign off would stand and likely another crew would take the aircraft. As I left the aircraft; I observed both mechanics might be preparing to tow or taxi the aircraft elsewhere. As we waited for aircraft Y; [I] observed aircraft X was no longer on the spot where I left it. As I waited for aircraft Y; I told the first officer what had transpired after he left. He told me 'yeah that mechanic was the same at the aircraft when I came back; that was prior to myself returning to aircraft' 'he; [the] mechanic; told me there was no leak.' I cannot comment what happened when myself and first officer were not attending the aircraft but think it is odd a leak I had observed and entered in aml no longer was present. All I know is I would have been delighted had I not observed the original or subsequent leak as we had 12-hour fdp (flight duty period) limited day and original scheduled xc:37 fdp. With that in the back of my mind it would have been convenient to accept the sign off without corrective action; but I felt there was a leak making the aircraft unairworthy. I have responsibility to the highest level of safety for my passengers; crew; and my company. I won't let that be compromised. It's also worthy to note that we blocked in XA37 with an original scheduled departure to ZZZ1 at XB00. Had we been given a different aircraft to begin with we could have done our original trip with a late departure and likely segments on time or very close. As I understand it aircraft Y was on the hardstand and available the entire time this event occurred. In the end due to additional ATC delays we didn't block in until XF53 and our crew was taken off our turn. I don't know if the flight was cancelled or operated by a different crew. Earlier I mentioned the technician told me all our [aircraft] had that particular leak. During my post flight on aircraft Y I noted all the 'banjo fittings were bone dry; clean and no evidence of a leak or previous leak for thatmatter.' this confirms my belief these types [of] leaks are infrequent and require corrective as soon as the leaks are observed.

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Original NASA ASRS Text

Title: Air carrier Captain reported refusing aircraft because of a recurring hydraulic leak and was challenged by Maintenance personnel.

Narrative: I went to the cockpit while FO (First Officer) did the preflight. Upon completing preflight FO brought to my attention a possible fluid leak top of left MLG (Main Landing Gear). I inspected the defect and noted fluid present on what I have come to know as 'banjo fitting' associated with brake lines on upper left MLG strut. FO pointed out stains on left MLG tires likely associated with wind catching the leaking fluid and landing on tire.I contacted Maintenance Control who requested the previously mentioned pictures. I also entered a write-up in AML (Aircraft Maintenance Logbook). Maintenance advised non-[company] on call Maintenance would be called. Approximately 30 minutes later; [Maintenance] arrived and confirmed the fluid present. After consulting with Maintenance Control; [they] advised me that I would be required to run the left engine for 3 minutes. It is my understanding [Maintenance] removed the fluid from the fitting I complied and determined that in feather I could get required pressure. Upon completion of the run; [Maintenance] came to [the] cockpit and declared there was no further evidence of the leak. I mentioned to [Maintenance] that I wasn't sure we accomplished anything as we never applied brakes or ebrake. He left the cockpit for a few minutes and returned stating Maintenance Control instructed us to run engine 3 minutes and actuate the brakes and ebrake a few times. At that point I called Maintenance Control and told them I would need to push back to do the engine run. Maintenance Control stated that was not necessary. I acknowledged their comments believing that was unsafe to do at the gate. I discussed with FO and determined if we captured with the tug and in feather; we could do the test at the gate. My belief was the test done in that configuration mimicked a typical push back operation. I coordinated with Ramp and proceeded to start number one and apply the brakes. At the same time; I took the opportunity to migrate brake fluid; 50% #1 75% #2; thus accomplishing the required Maintenance request. I quickly realized with ebrake pressure being depleted #2 would need to be started. That was accomplished and Maintenance request was completed. With engines in feather I was able to get adequate hydraulic pressure to accomplish the Maintenance request. Had we needed to come out of feather we would have pushed back and accomplished on a taxiway.[Maintenance] did a general visual inspection and determined no further leaks. It is worthy to note no actual maintenance procedure occurred other than cleaning the fitting and running the engine. AML was signed off and we departed for SEA.Upon arrival in ZZZ; FO left the aircraft to get some breakfast. I did the required post flight and upon inspecting the above mentioned 'banjo fitting' I observed a yellowish fluid evident on the connector. The amount was 2 drips vs the 3 drips observed on the original AML entry. I called Maintenance Control and made another entry into the AML. At that point I left the aircraft for about 15 minutes to go get a coffee. I returned to find a Maintenance Technician holding the book and as I recall in the vicinity of the L landing gear. I inquired of the status and he indicated the AML entry was signed off with GVI (General Visual Inspection) no leaks noted. Thinking that was odd; [I] proceeded to observe that [the] connector was in fact clean and free of leaks. I mentioned that someone must have wiped it off. The Technician told no one had. At that point knowing the connector had a leak that reoccurred I told the Technician I would not be able to accept the aircraft. As I recall his response was something along the lines of OK. I called Maintenance Control and explained the situation and they advised me the aircraft was signed off and good to go. I explained to Maintenance Control I was going to refuse the aircraft. Maintenance Control advised me I would need to contact the FODO (Flight Operations Duty Officer). I called Dispatch and he sent a request to FODO to call me. Shortly thereafter FODO contacted me and I explained the situation. He indicated he would contact [Maintenance Operations] and Maintenance Control. [FODO] called me back and relayed a request from Maintenance Control that I fly it just one more leg and they would change out my airplane. I declined as I believed there was a leak that required corrective action before another flight. [FODO] indicated he understood my position and would attempt to secure a different aircraft to finish my day. [Then]; I was advised I had a different aircraft; Aircraft Y. Shortly after that I believe the crew started packing up to go to Aircraft Y on spot 3. Sometime before the FO departed to Aircraft Y a different Technician along with the original Technician that I had interacted with came on board the aircraft. He asked me to point out the leak that I had written up. I proceeded to the left engine and pointed out the connector that was at this point clean and dry. He proceeded to tell me it was clean and no leak was noted. Additionally he pointed out these leaks; presumably ones that I had written up; were on ALL the airplanes and are very common and that's just how those connectors are. He told me that he had not wiped that connector off. Which was interesting as I didn't say or infer he did. He proceeded to then tell me the other side was the same way and 'Why didn't you write that one up.' I began to get a feeling this might escalate into an undesirable exchange so I told that Technician 'Thank you for your observation' and began to collect my belongings for the plane change. After that exchange I observed the two technicians take the AML and enter the walkway out of earshot and examine the logbook and appeared to be making an additional entry into the AML. Before leaving to Aircraft Y the AML was replaced being the Captain seat. Curious what the additional entry might have been; the last entry was the sign off from my original writeup. I got the distinct feeling that the sign off would stand and likely another crew would take the aircraft. As I left the aircraft; I observed both mechanics might be preparing to tow or taxi the aircraft elsewhere. As we waited for Aircraft Y; [I] observed Aircraft X was no longer on the spot where I left it. As I waited for Aircraft Y; I told the FO what had transpired after he left. He told me 'Yeah that mechanic was the same at the aircraft when I came back; that was prior to myself returning to aircraft' 'He; [the] mechanic; told me there was no leak.' I cannot comment what happened when myself and FO were not attending the aircraft but think it is odd a leak I had observed and entered in AML no longer was present. All I know is I would have been delighted had I not observed the original or subsequent leak as we had 12-hour FDP (Flight Duty Period) limited day and original scheduled XC:37 FDP. With that in the back of my mind it would have been convenient to accept the sign off without corrective action; but I felt there was a leak making the aircraft unairworthy. I have responsibility to the highest level of safety for my passengers; crew; and my company. I won't let that be compromised. It's also worthy to note that we blocked in XA37 with an original scheduled departure to ZZZ1 at XB00. Had we been given a different aircraft to begin with we could have done our original trip with a late departure and likely segments on time or very close. As I understand it Aircraft Y was on the hardstand and available the entire time this event occurred. In the end due to additional ATC delays we didn't block in until XF53 and our crew was taken off our turn. I don't know if the flight was cancelled or operated by a different crew. Earlier I mentioned the Technician told me all our [aircraft] had that particular leak. During my post flight on Aircraft Y I noted all the 'banjo fittings were bone dry; clean and no evidence of a leak or previous leak for thatmatter.' This confirms my belief these types [of] leaks are infrequent and require corrective as soon as the leaks are observed.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.