I've read all the brake fluid communications. There still seems some confusion that does need clearing up, or mistakes might happen. If you know all, this you can stop reading and press delete. If not then read on...
There are six (6) different types of brake and hydraulic fluid. Each of them are made of a slightly different base material and have to be used with the appropriate rubber seals. Use of the wrong fluid in the wrong system could mean that the rubber seals will expand, either leading to a leaking master or brake cylinder or seized piston cylinders. Which is messy! The six fluids are: Skydrol. Ester based, often clear or slightly yellow or died Purple, used by airliners. It doesn't freeze at altitude and is fireproof, but it is poisonous and is bloody horrible stuff to work with. I flipin hate it. The safety gear you have to wear is necessary. If you don't think so, put some on your hand and see how long before your hand falls off. PTFE seals needed, shelf life 5 years and then lasts forever in the plane or depending on what the MM says to change it, but you won't be using this on your homebuild. Aeroshell Fluid 41. The Pink stuff. You could drink it if you so desired. Mineral-based. Very stable. You need to use synthetic types of rubber seals and NOT natural rubber. It doesn't like that. Shelf life is 3 years, but that is because they want you to buy more every three years. I still have some of the old Fluid 4 left over and it's perfect. We have worked on aircraft that have been stored forever and when returning them to service, yes you change the fluid, but what comes out looks like the day it went in. Then there are the car fluids. DOT 3, DOT 4, DOT 5 and DOT 5.1. They are all the same. Glycol-based, clear fluid, but DOT 5 is not. Dot 5 is Silicone based, doesn't absorb water, is very stable but is mostly used on bicycle brakes. The other DOT brake fluids absorb water readily. It strips paint, but if you spill it just quickly wash it off, it is highly water-soluble, but kills plant and animal life if you wash it down the drain. If there is no chance of water contamination, either in storage or in your car brake system, you do not need to change it every 2 years as recommended by the people who want to sell you more. If you are in doubt of the condition of your brake fluid, change it, if you think it is ok. Leave it. Brake fluid does not deteriorate as quickly or as bad as made out. If you get it highly contaminated with water, by leaving your cap off and pushing the car out in the rain for years, then there is a possibility of you boiling the water in the brake cylinders, but really?I don't think so! When the brake fluid goes dark or brown, it is actually bits of the rubber seals. Yes, they do wear away when used, as the seals obviously move forwards and backwards in the cylinders. So yes your fluid will slowly change colour, but it's the rubber seals that are wearing away, not your brake fluid going off. Then you have the rubber seals. You have Natural Rubber from trees, Latex rubber. Used in very very old brakes. Doesn't like petrol or ethanol or many fluids. The Glycol brake fluids are best. You have your Nitrile Rubber seals. You can spill petrol on them but it deteriorates at high temperatures. Not good for brakes but FLUID 41 is ok or any of the Glycol-based brake fluids. You have the Viton seals and Flurosilicone seals, same as Nitrile Rubber, but better with high temperatures. Mineral fluids or Glycol are fine. Then you have the best seals for brakes. The EPDM high-temperature synthetic rubber. All fluids are ok with this, but then strangely don't spill petrol on them or wash aggressively with a petrol spray as they will swell up and go soft. Most old European aircraft used French Car Brakes, which had Natural Latex Rubber seals, so you have to use DOT fluids, not Aeroshell, it is ok, but really you should use DOT fluids. Most modern aircraft use Cleveland Brakes which require Aeroshell fluid 41. But if you put DOT fluid in the system, it buggers the seals not only at the brake calliper seals but the brake pedal seals up by your feet. Some people upgrades from Drum Brakes to Cleveland Disc brakes. Then the Master cylinders by your feet needed Car brake fluid and the Brake disc cylinders needed Fluid 41. So that won't work. Mix Fluid 41 with Dot fluid, you get instantly a brown gooey mess. SO, conclusion, you really have to go the extra mile and find out exactly what brakes you have, what rubber seals are used, not only in the brake cylinder, but the master cylinder also and what fluid you need. Don't guess The fluid is good for many many years, as long as the system is well maintained with no chance of contamination. Put new fluid in all the time if you want, but if you haven't got new fluid and it looks clear, it's okay. CH.
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