When my car is off I can pump the clutch pedal and have pressure to disengage the clutch. When the car turns on I lose almost all hydraulic pressure for my clutch system. For your information if it helps you diagnose my problem. I just replaced my slave cylinder when I put in a new clutch and flywheel a few thousand miles ago and I replaced my master cylinder a few days ago. When I replaced my master cylinder I replaced it with a 13-14 GT500 master cylinder and during the install I realized my hydraulic fluid was darker and cloudy. Ps. I replaced the master cylinder because I wasn’t being able to disengage my clutch enough to even shift gears. That started a few day after I let out my clutch before I was completely in gear and my gears binded and the transmission made a loud pop noise. (Stupid mistake) Although after the new clutch master cylinder install my car shifted smoothly and the clutch pedal was firm for two days. Then all of the sudden I can build clutch pressure but can’t keep it when the cars on? Idk. Please help! Thank you!