I don't know what you mean by clutch grease, but the clutch master cylinder uses dot 3 brake fluid as a hydraulic.
You ONLY use the clutch brake when you're at a complete stop.
Assuming you mean "engage the clutch" or "Release the clutch pedal" which is the exact opposite of what you actually asked: 1) clutch disc contaminated with grease/oil/brake fluid 2) engine misfire (is RPM high enough?)
grease or oil on the clutch facing
To replace the clutch brake on a peterbilt you have to remove the inspection cover on the bottom of the bell housing. You then have to determine whether you have a two piece or a one piece clutch brake. If it is original it is probably a one piece. A one piece will have to be cut off usually with a torch. Be careful because very hot grease can fall in your face. A two piece clutch brake has a hinged side and a push lock on the other. the side with the push lock can usually be released with a screw driver. If the transmission is not removed a two piece clutch brake will have to installed on the input shaft.
NO! No...no...no...! Do not put any grease on your brake discs, copper grease is used to put on the 'back' of the brake pads (not the part of the pad that comes into contact with the disc).
It uses brake fluid in the brake fluid reservoir. The clutch fluid (brake fluid) is fed by the brake fluid reservoir. The clutch fluid reservoir is just beneath the brake fluid reservoir. So, add fluid to the brake fluid reservoir to fill the clutch fluid, also.
Brake, emergency brake and the clutch
A blown out rear axle seal will cause grease to leak out into the brake drums
grease or oil on the clutch facing
Yes, there is no such thing as clutch fluid. It's all brake fluid.
Brake Fluid Dot 3 Note: Brake and Clutch fluid are the same