First of all it's very important to properly clean the brake hardware
. And brake parts cleaner is very effective at doing so without leaving a residue
. So if you have grease, oil , dirt or whatever you have to clean them off properly using brake parts cleaner
The reason the label says to protect rubber is that petroleum solvents are soluble in rubber, but are not instant rubber killers. What happens is that they break down the molecular chains and the rubber swells as the chains are broken down by the solvent.
While I personally don't use break parts cleaner to clean off the top of the master cylinder reservoir (as I said, toothbrush and alcohol and let dry), I'd point the brake parts spray away from the rubber (such as the pads and bracket rails where the pads sit) and clean the daylights out of them. Because you don't want grease/oil or the pads to stick on caked in dirt and you can do so aiming away from the anchor pin boots for example.
You can also dip a copper wirebrush, steel wirebrush, or toothbrush in some brake parts cleaner and scrub at caked in dirt/brake dust. You can wipe the anchor pin boots/piston dust boot areas with towel and alcohol. At least that worked for me. If necessary cover the pin boots, but I've not found it necessary.
That's another reason why you don't use coolant hoses on ATF lines. There is a rubber formulation for each application, and I specify the application at the NAPA parts counter. That's why 3/8" ATF hoses are like ~$5/ft vs ~$1/ft for similar coolant hoses. And AFAIK
, brake system rubbers aren't as resistant to petroleum oil as ATF hoses.