My clutch works just fine 99% of the time, but when I have to hold it down or feather it, I think some fluid is leaking past one of the pistons. It happens when I'm backing up my driveway and into the garage. When I stop, I have the pedal all the way down, but the clutch is definitely a bit engaged as the engine shakes slightly and it's hard to get out of gear. Then when I turn the car off and lift my foot, the pedal comes halfway up, and I have to pump it to get it to come all the way.

Based on everyone's experience, is it more likely the master or slave cylinder seals, or maybe something else entirely?

2005 R52 S, ~75000 miles, I've owned for less than 10,000.

 

Based on my experience, I'd bet on the slave, mostly because I've never replaced a master cylinder, but I have replaced a slave, because it was leaking, but working just fine. Your problem sounds different from my experience. If I were you, I'd check your brake fluid level, and look at your slave cylinder to see if it's leaking, bleed the slave, and if it's still a problem, replace the slave (and bleed again) If it's still a problem, replace the master cylinder and see what happens.

Just know that bleeding the slave cylinder is a bit of a pain and requires a special tool or a little ingenuity. I'm a pretty fair mechanic, but I opted to pay a guy $100 to change my slave cylinder because I didn't want to deal with it. Also if you do change the slave cylinder, be very careful not to break off the tabs on the transmission that the cylinder bolts to. Use plenty of penetrating oil, heat and patience to get the bolts out.

 

Reservoir was closer to the MAX line than the MIN line, so that's not the issue. Replacement OEM slave cylinder, fluid, and pressure bleed adapter arriving tomorrow from Amazon. The ModMini video tutorial is great.