Standard oil grease as Shimano Roller Brake grease short-term replacement

Question

I am currently in the process of cleaning up the rear axle from dirt and excessive grease, legacy of the previous owner. I will be replacing a ball bearing and few other small components, but would like to avoid any costly repairs (this is a pretty worn down bicycle after all). The previous owner put copious amounts of grease in the rear brake (Shimano BR IM 55 R), so much that it started leaking out of the brake. I want to clean the break of all grease altogether, and put new Shimano brake grease (original brand). However, it will arrive only in few days and I really need my bike as it is the only mode of transport here (I don't own a car). Can I put some cheap general purpose bike oil (picture below) instead for just few days, and then remove it once more and replace it with the intended grease? (I can refrain from using rear brake for those few days if deemed necessary).

Answer 1

The Shimano roller brake design is pretty robust, but I think replacing the grease with any kind of oil would be a good first step to making things actually go wrong with one. The consistency of the lubricant in there is likely going to be the factor that matters most in making it work right, and an oil is much lighter than the Shimano grease.

Shimano roller grease itself is on the somewhat light side. Common bike greases are NLGI 2, and Shimano roller grease feels like it's in the 1-ish range. It's like a yogurt to NLGI 2's Vaseline. The intention there is likely something along the lines of having it flowy enough to get where it needs to be, but thick enough to be durable under friction and heat, and not run out of the hub. From the position you're in, if you were to take out everything in there and replace it with oil, I think you could stand to risk it being under-lubricated in use. Having an oil do the job of a flowy-leaning grease could cause the braking surfaces to effectively run dry in use, since at the actual points of friction it might break down quickly, and the nature of oil compared to grease in this application (where gravity wants to leak it out of the unit) is that there may not be more around to flow in and replace it. You don't want that.

Your language suggests you have the means to take out what's in there. If that's true and if it's basically working but just messy from being over-lubricated, take out some of what's there and call it good.

Answer 2

Non-Shimano grease will very likely degrade braking power, possibly to the point of being dangerous (speaking from experience).

In principle, the Shimano roller brakes are pretty robust and unless the previous owner was routinely riding through puddles of saltwater or something similarly extreme, I wouldn't expect the inside of the brake to be actually contaminated. If it rotates fine, brakes fine and you don't hear or feel grinding, chances are, the insides are okay. Just wipe the excess and leave the brake as is.

Incidentally, Shimano does not consider those brakes serviceable, they are made to be replaced as a whole unit. It is in principle possible to open, clean, and rebuild the brake, but unless you are very experienced at this kind of work, I would not touch it, chances are you won't be able to put the brake back together again.