C172S pilot reported a fuel sump drain could not be reseated while performing the preflight check. After completing the flight; the pilot found out that the aircraft had a history of leaking fuel sump drains.

Narrative

After arriving at the cross-country destination in my flight school's Cessna 172S; I had the FBO refuel and fill up the tanks. I sumped the 13 fuel sump locations after checking the level with a fuel stick as per the checklist. Upon collecting the fuel sample from the last left-wing fuel sump; the o-ring would not reseal correctly. After pushing the needle into the sump gently; collecting the sample; and pulling the needle out; as I have always done; straight up and straight down with no excessive force; fuel began to leak out of the sump nozzle in a continuous stream. I finessed the needle of the sump cup in and out to seal the sump as best as I could but it was still leaking.At this point; which was after closing time for the FBO with no maintenance staff; I video-called my Commercial Pilot's License Instructor on my phone's [video calling] to ask for help with reseating the o-ring and to show him video evidence of the situation. He instructed me to continue to attempt to reseat the o-ring with the fuel sump cup needle. After retrying to seal it; the leak had begun to stream from the sump again as opposed to the drip I managed to reduce it to. He immediately hung up with me to call the owner of the flight school to which the plane belonged. My instructor called me back and told me that the owner wanted me to fly back to the home airport; as-is. My instructor was adamant that this was my decision and stressed that it was up to me if I felt comfortable with the situation as pilot in command. A first-hand witness of the owner's side of the conversation; who was in the room with the owner when he was on the phone with my instructor; back at the school; confirmed to me that the owner was resolute and uncompromising on the matter; fully intent on me to fly the aircraft back with the leak. Whether it was a demand or not; I'm not entirely sure. Again; my instructor relayed this to me but was clear that if I felt uncomfortable or unable; that was my choice. My instructor is a considerate man with strong moral integrity; a combat US Army veteran with the utmost respect for safety and regulation. I want to be very clear on that point. While still on [the video call] with my instructor I managed to seal the sump enough to contain the leak so a very slow drip. By now; the leak was reduced to about a single drop every few seconds. I told him I would let him know of my decision.This is where my ASRS report becomes important. I exercised poor ADM and decided to fly the plane back home. I rechecked the level of fuel in both tanks and confirmed with the fuel stick to still have 26.5 gallons in each tank. When I left the home airport there were 50 gallons onboard; and when I refueled; 13 gallons were administered. Meaning that; with a headwind; the flight; including taxi; run-up; takeoff; and climb-out consumed 10 gallons with a 50-degree rich of peak EGT mixture with an actual time enroute of 1.2 hours. Knowing my destination was 110 NM away; with a calculated KTAS of 110 kt. and a strong tailwind; I would be at my destination in; conservatively; one hour. With an overly conservative fuel burn rate of 12 gallons per hour; 3 gallons for taxi; run-up; takeoff; and climb-out; which is double what the POH specifies; and considering an extremely conservative estimate of leaking 5 gallons per hour; I would make it back home well within legal VFR fuel minimum requirements. The leak rate was nowhere near this high. I knew that I could make it back with the fuel entirely in the right tank alone; if necessary. Especially if I ran the mixture at peak EGT. I would be flying a straight-in approach to a runway aligned with my route of flight and my route took me directly over an alternate airport; with a runway aligned with surface winds and pilot-controlled lighting; as well as along paved; lighted; wide stretches of highway; in a plane that was equipped with G1000 avionics containing calibrated fuel level sensors and a fuel flow rate gauge. Then after considering the amount of leaking fuel; the location of the leak; i.e.; on the aft underside of the wing with no electrical or mechanical equipment behind it; and what kind of fire hazards it might present if any; I determined the minuscule leak was of an inconsequential nature to the safety of flight. I contacted my instructor via text to inform him of my decision and began following the checklists for departure. I climbed out and requested VFR flight following with Center frequency. Upon being instructed to contact Approach for my destination; I informed the Approach Controller of a small fuel leak and requested to expedite the approach for a straight-in landing on my desired runway. After doing so; ATC asked me if I knew where the leak was coming from and I informed them that it was insight and coming from the left wing sump nozzle. At this point; they gave me priority handling; said that they were rolling the fire trucks out to the runway; and asked for remaining fuel and souls onboard. I landed the plane without incident and was escorted back to the flight school's Ramp by a fire truck. The owner was outside the hangar waiting for me with a tow bar and bucket to catch any leaking fuel. I checked the level in each tank after shutting down and found 17 gallons in the right-wing tank and 15 gallons in the left-wing tank with the leak. This was a life lesson I won't soon forget. Aside from never doing something like that again; it has taught me that I can truly only rely on myself for taking my complete safety into serious consideration because the school owner didn't seem to mind much knowing there was a leak and insisting I fly home like that. After telling my wife what had happened; she was beside herself with my lack of judgment in putting our mutual future together in jeopardy. It prompted me to retake a hazardous attitudes inventory test and; after finding out later that that very plane has had the same exact fuel sump nozzle leak profusely; to start looking for a new school to finish my CPL and to later instruct for. Neither I nor my instructor knew that this plane had experienced that issue in the recent past. I was not informed of it before renting it.