Brake fluid will eat the seals in your clutch perch lever device. Mineral oil is red or purple. Sounds like maybe the lever piston is not functional now.
Thank you so much.... is this Green LHM+ something like the VW Power Steering Fluid which is also mineral oil???? I have already ordered Shimano fluid but would be nice to know if this other fluid could be compatible. For my Audi I use CHF22... green mineral oil.Black fluid in the reservoir on any bike tends to indicate the master cylinder seal has degraded (fine black rubber particles in the fluid). My K1300GT had pitch black fluid in the reservoir when I got it 5 years ago but I replaced it with Vitam LS (the BMW red stuff) and it worked OK. It came out darkish again once more and, after using up the small bottle of Vitam, I started using the green LHM+ (Light Hydraulic Mineral Oil) designed for car hydraulic steering/suspension and brake systems (e.g. LHM Plus Hydraulic Fluid - as you can see its cheap) and it's been fine ever since. Best to use the "+" version as its pour point is lower.
I wouldn't assume it's got brake fluid in - it could be that someone used an ordinary mineral oil. If you can bleed it up OK (bit of a faff - definitely best to back-bleed by pushing up with a syringe from the bottom - and you need to replace the blanking screw with a nipple (like this Clutch slave cylinder bleed nipple) and cap) and get it working properly, and it stays working for a couple of months, you may not need to worry about damaged seals.
I think most of the "CHF" products are synthetic oils. I don't know specific reasons why they can't be mixed with mineral oil (like you can with mineral and synthetic engine oils) but there're mixed opinions in the car forums about it, so probably best avoid it just in case the one you pick is somehow different. Don't know about the VW stuff but suspect, if its definitely a light mineral hydraulic oil, it should be fine. I use LHM because its definitely a light hydraulic mineral oil (the clue's in the name!).Thank you so much.... is this Green LHM+ something like the VW Power Steering Fluid which is also mineral oil???? I have already ordered Shimano fluid but would be nice to know if this other fluid could be compatible. For my Audi I use CHF22... green mineral oil.