Most minor, single mouth ulcers are caused by damage to the mouth, for example by accidentally biting the inside of your cheek while eating, or from a sharp tooth, food or filling.
I have a 2005 Honda Element. The owners manual states the brake fluid should be replaced every 3 years. When I asked the dealer to replace the brake fluid, they said it was going to cost $190. I told them last time they did it, it was only $27.
CHANGE BRAKE FLUID. When the fluid is changed, use the type of brake fluid (DOT 3 or 4) specified by the vehicle manufacturer. The cap on the fluid reservoir will usually indicate what type of brake fluid is required. You can also find this information in your Owners Manual (look under brake fluid).