Recently I've noticed that every so often, the clutch seems to engage closer to the floor than normal. No other change in clutch feel and weight. I'm suspecting the slave cylinder is going bad because the car has 155k miles on it, and it's never been replaced.

Weird thing is, I don't see any fluid leaking. The brake fluid level in the reservoir stays consistent.

Can the slave or master cylinder still be bad? How would I know for sure if there are no visible leaks?

Also the clutch was replaced about 15,000 miles ago.

 

No MINI experience with its clutch hydraulic system but with another brand of car I can tell you if the clutch hydraulic fluid is brake fluid (gyroscopic) a clutch hydraulic system flush/bleed -- like a brake hydraulic system flush/bleed -- can work a minor miracle in addressing untoward clutch (and transmission shifting) behavior.

Actually the car above the clutch hydraulic system shared fluid with the brake hydraulic system. While the brakes were not manifesting any signs of any issues the clutch was and as a result -- as it turned out -- this was affecting shifting.

Will add with my 1st JCW at around 1 year into ownership and with around 15K miles on the car the rear brakes needed replacing. This was a shock to me as brake life with my other cars was very good. Had the car for new rear brake pads/rotors, but didn't request a brake fluid flush/bleed. Guess I believed at only 1 year in service this was unnecessary. After new brakes I noticed the stability control activating and doing so clearly when it was not called for. During one nice long sweeping right hand turn I took almost daily on my work commute I could feel the driver side rear brake being activated. This happened to be the worse of the two rear brake systems.

On a hunch I had the car in for a brake fluid flush/bleed. After no more over active stability control system.

Before you condemn the master/slave cylinders do a clutch hydraulic system flush/bleed if possible. And if the brake and clutch hydraulic systems share fluid do a brake hydraulic system flush/bleed *first* then follow with a clutch hydraulic system flush/bleed. This is the sequence called for by the factory for my other cars. 'course, you should know what sequence MINI calls for and follow the recommended sequence.

 

I see. Yeah maybe I'll try flushing/ bleeding the brakes first. I'm a little nervous about bleeding the clutch though because I heard its a PITA in these cars.

Is the slave and master cylinder not something you should preemptively replace?

 

Well, having just recently preformed a full hydraulic/brake fluid flush and bleed for the first time on a Mini/modern car, for the same reason's you experience - It wasn't bad at all. In fact it was imperative for my case.
I was beginning to think I needed a pressure plate or entire clutch replacement given my pedal travel didn't feel right and hard shifting after a while. Although I thought it unlikely since the clutch disk had been replaced only 25k ago.
Here is the link to my post: "Hydraulic flush..."

Long story short:
My hydraulic/brake fluid was long overdue for a change.
Having to resort to doing a flush by myself = I bought a pressure bleeder.
Bled all the brakes til all the air was removed and fluid ran crystal clear.
My original clutch slave cylinder looked very good and wasn't leaking so I was lucky enough not having to replace it or remove it. I just bled the heck out of it (6oz) - installed in-place on the car. .
After completely flushing the entire brake & clutch slave; resulted in getting my pedal back. I now have about 1-3/8" - 1. 5" of travel while starting engage/disengage from the floor and completely engages or disengages 2' thereafter.

One thing to also note: If you preform the flush properly (getting all the air out), and regain a better pedal feel. Be sure to place a long (2') piece if wood between the clutch pedal and seat track, leaving the clutch pedal pushed all the way to the floor overnight.
The next day, hold the clutch pedal to the floor as you remove the length of wood and then ever so slowly let the clutch pedal up.

There is a fair wealth of good info written by contributors here on NAM. You can google Mini clutch slave bleed + NAM in your search query for better results.

Best of luck!

EDIT:
BTW - I give full credit to @RockC for suggesting a complete flush and bleed that solved a majority of my stiff shifting and low clutch pedal feel.

 

Don't know about the MINI clutch hydraulic system flush/bleed procedure. I did my Porsche Boxster and used a power bleeder. I had no one to push the clutch pedal down and hold it while I opened the slave bleed screw.

While it was not a difficult or tricky job I didn't do something right and the next day at the office I had to enlist the help of a co-worker to work the clutch pedal while I opened the slave bleed screw. All it took was a couple of times. The clutch was fine after. The next time and subsequent times the brakes (and clutch) needed bleeding I had the car into the dealer.

To just out of the blue replace the clutch master cylinder/slave cylinder is a bit out of the ordinary. Not to beat you up but if you think the clutch master cylinder and slave cylinder need preemptive replacement what about the even more critical brake master cylinder and all the slave cylinders which happen to reside in the calipers?

That being said my auto tech buddies impressed upon me that sometimes things get replaced while one was is "there". Thus it might be when one does a MINI clutch this might be the time to replace at least the slave cylinder. But I rely upon a tech's opinion. If he says Yeah these need to be replaced when a new clutch is installed then I have him do it. But if he says absent any signs of any problems leave it be, then I agree and he leaves it be.