[Jeepophile]

In over 45 years of vehicle ownership I have flushed my brake systems every one to two years; as a result, I've never had to replace a master or wheel cylinder on any vehicle I bought new---only made such replacements on a couple of vehicles I bought used. I use a one-man pressure bleeder with an air bladder to keep air from being compressed into the fluid. My brakes have always performed well in severe/emergency situations, and I'm glad I've done the flushing on a regular basis. As I bought another vehicle, I bought another adaptor if necessary so that the one-man bleeder can be attached to that particular vehicle's master cylinder. One needs to take measures to exclude air from the fluid when refilling the bleeder, as well.

If you pay to have the system flushed, make sure to at least check the master cylinder reservoir to sort of see that the new, clearer fluid is present; my wife's
(she was at that time my new girlfriend) vehicle was supposedly flushed by the Acura (!) dealer---when I checked it, the master cylinder reservoir appeared to still have brownish fluid---I complained to the service manager, and the mechanic said he "forgot"....

 

[KOG]

I flush brakes every two years.

 

[ImperialCrown]

All fluids do wear out. Brake fluid (hygroscopic) absorbs water over the years and the boiling point lowers. It is Glycol and not petroleum. Moisture in the fluid will form acids and such over time that will attack (swell) rubber seals and hoses. It will also corrode wheel cylinders and calipers.
Taking apart old leaky wheel cylinders, you may find pitting at the bottom of the cylinder bore that provided a leak path. That pitting probably started with moisture in the fluid.
Sometimes stuck caliper pistons get that way from corrosion starting from the inside out. Or if the boot lets in moisture from the outside in.
We change engine oils and occasionally coolant and transmission fluid services, but items like brakes, differentials, transfer cases and power steering are generally forgotten.
How often? I think that at least every 60K miles (96,000km) or when ever it may be convenient like during a pad or shoe replacement or other brake service when the wheels are off.
The hardest (and scariest) part may be cracking the bleeder screws loose. A couple of counter-clockwise 'jerks' or 'batting' the ratchet handle with your open palm helps to get them loose rather than a steadily applied force. I hate breaking off bleeder screws.
Use a 6-point socket and not a 12-point socket that can round off the corners of the screw head. If that happens, your 2nd chance to loosen them is with vise-grips. At that point remove and replace the bleeder screw. Many auto parts stores carry generic ones.
Some have rubber caps over the bleeder fluid hole. Many times these are removed and never reinstalled. They help keep out water and dirt and are important to keep the bleeder hole clear.
Gravity bleeding can be done by just letting the fluid drain out of an open bleeder. Having a helper push the brake pedal is better. With their foot on the brake, open the bleeder, the fluid should spurt and the pedal fall to the floor (another scary sensation). The helper needs to keep it on the floor until you close the bleeder. Then you can signal them with an 'OK' to resume a couple of pumps of the pedal to do it again. Keep this cycle going until the brake fluid runs clear, then move on to the next bleeder.
Brake fluid isn't an oil and should rinse off with water. It can attack asphalt and soften tar, so if you pride your driveway, lay down cardboard or rinse off the spills after the fluid maintenance. The brake fluid can wrinkle the skin on your fingers, so either wear rubber gloves or wash with soap and water afterwards. Have rags handy for this work and use safe jacking and vehicle support practices.
Only choose a fluid that is compatible with your vehicle in your owners manual information. There are many choices. Read the label. I see some advertised as being 'synthetic' and that is OK as long as it is compatible with what is in your vehicle.
DOT 4 is a heavier-duty fluid than DOT 3 and are compatible. It is OK to use DOT 4 in a DOT 3 vehicle, But never use DOT 3 in a DOT 4 vehicle.
Silicone brake fluids (DOT 5) are overkill for most regular vehicle applications and may be OK for racing and heavy-duty applications. It must only be put into a It has no trace of other DOT fluid in it. It has its advantages and is more expensive.

 

[dak4x4]

I have never flushed brake lines even though it is sorta universally recommended. Have kept some vehicles almost 20 years and never had a brake problem other than worn out pads or shoes or rotors..

Back when I was a pup shade tree mechanic, I had occasion to bleed brakes, and kept getting soft pedal and air in the line; this cured me of messing with bleeding brakes for life.

 

[KOG]

Brake bleeder screws are a great place to use CRC Freeze Off. Put the straw down the center of the bleeder and freeze until screw loosens. Works like a charm.

 

[Bob Lincoln]

I've never flushed fluid until it was time to do service. Usually it's been fine, on occasion I've seen some yellow-orange tint to the fluid, so I flush until it's clear. My last car once put out 'ink', but in 308K miles, I never had a leak or failure. But it is good to change it once in awhile, depending on how you drive and how far.

 

[68RT]

I can see a 2 year flush on units that do not have the sealing bladder on the fluid reservoir. Since that addition, the need for frequent replacement has gone way down. As long as the bladder is sealed, I would not do it until brake service is needed personally. Older cars without the bladder shoul be changed at about tow years if in an area that has fair amounts of rain or high humidity.

 

[KOG]

All of mine have the bladder and fluid still goes dark after a couple of years.

 

[Jeepophile]

As a last step (before I cover cap off the bleeder screws--if they have caps) after flushing and final screw micro-torque, I spin in and out several dry little cones of paper towel to dry out the fluid remnant on the atmosphere side of the screw; otherwise I always got crud in that space that contributed to screw corrosion and/or difficulty getting the fluid to come out next time. Not worried about the slight amount of paper towel lint that might be left there, the fluid should push out any remnant next flush.

 

[AC TC]

I have my own way wich is someting in the middle.
i change the fluid in the master reservoir every oil change and every brake pad change.
it isnt a real flush but it keep things looking ok.
I do the same with the servo oil also, here its easy to se the results.

 

[Jeepophile]

I assume you mean you syringe out fluid from master, then add fresh, Be careful not to add with suction bulb whereby you can press air into the fluid and thus into brake system.

 

[AC TC]

I´ve got a "vacum" can with a hoose, great for all kinds of fluids.
I dont suck at the port for the mastercyl, rather i vacum the bottom with all its debris.
The key here is to remember to fill the master before braking and leave the new fluid to deaerate for a couple of minutes
after filling.
Im also careful to clean the hoose before inserting since i dont what to introduce oil in the brakefluid.