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Thread: How often do you bleed the brakes (flush the system)?

  1. #1

    How often do you bleed the brakes (flush the system)?

    My brakes work great. The rear wheel cylinders and all shoes, the wafer spacers, etc., were replaced 3 years ago, all brake lines replaced 6 years ago, Front wheel cylinders are from 1989., master cylinder I replaced in 1975. I've never touched the booster. The last time I bleed out the brake fluid with Dot 5 was in 2004. Although the level in the cannister drops very very slowly, it has turned brown. How often do you guys bleed out the old fluid, should it be once a year? Frank

  2. #2
    Administrator JimVillers's Avatar
    Join Date
    Aug 2001
    Virginia Beach, VA
    Frank .... My brake system gets flushed about every third year. For years, I did not think that it was important until I started doing track days with another car. For track events, many organizations require the fluid being changed in the previous three months. After seeing a car that had not had the fluid flushed boil the fluid in the calipers, I became a "flushing fan". It is hard to comprehend how much water can get into a brake system to either cause corrosion or to boil during extended hard braking. The less the car is used, the more important it is to flush the brake fluid and to cycle the cylinders.

    You mentioned silicon fluid and I don't know where the water goes with silicon fluids or what corrosion issues arewith silicon.
    Jim Villers
    1961 190SL, 230SL 5-speed, MGB 5-speed, Boxster 'S'; Porsche 356C; 1967 Porsche 911; 1950 Jeep CJ3A; 1958 190SL "Frosty"

  3. #3
    Registered User
    Join Date
    Mar 2008

    Flush it down

    I also had a embarrasing situation from old dirty brake fluid ,my both rear brakes ceased on the highway,
    My mechanic had to blead the brakes to get them to release,he said a tiny piece of dirt blocked the fluid return.
    So now my rule is 1, 2,or3 years .But more important.Treat it like a poor septic system at the cottage, if its yellow let it mellow but if its brown flush it down
    Jim Rombough
    1960 190 sl

  4. #4
    23K Original Miles slover's Avatar
    Join Date
    Jul 2005
    Quote Originally Posted by JimVillers View Post
    You mentioned silicon fluid and I don't know where the water goes with silicon fluids or what corrosion issues arewith silicon.
    Short answer, nowhere, unless it's turned to steam/air from boiling. Corrosion issues less with Silicone since it doesn't attract moisture.

    Silicone based fluids (Dot 5) are non-hygroscopic, they will not absorb or mix with water unlike Glycol based fluid (Dot 3,4,5.1). When water is present in the Silicone brake system it creates an unmixed water/fluid/water/fluid situation in the brake lines. Water boils at approximately 212 F, and the steam created from boiling water adds air (very compressible) to the system in place of the boiled water and therefore an increasingly spongy pedal occurs. Silicone brake fluid lacks the ability to deal with moisture unlike Glycol based brake fluid which absorbs the water with the resulting negative of a lower overall boiling point("wet," 3% water), but higher than the plain water in the Silicone system. Racers do not use Silicone because it is more compressible at higher temps (like when racing) and results in too spongy of a pedal, Glycol is two times less compressible. Silicone has a longer retention of suspended air especially micro-aeration introducing air to the brake system during bleeding and possibly requiring multiple bleedings.

    With respect to water in the brake system, Glycol based fluid is hygroscopic, it attracts and mixes with water. Therefore, poorly sealed and older bottles of Glycol fluid, poorly cleaned bleeding tools, a poorly sealed reservoir and too long a period of an open brake system (like when bleeding) will attract moisture from the air (think Virginia summers) in a system with Glycol based fluid. Heating and cooling calipers can result in water condensation. Water can also enter through brake hoses over time. To minimize moisture attraction in Glycol systems, use fresh fluid, do not store fluid, use a sealed pressure based bleeding system, like Motive that Jim Villers uses, that seals the brake fluid reservoir while bleeding and work on a dry day.

    Glycol based fluid ruins paint, Silicone does not.

    Bottom line, pick your poison and bleed your system often (yearly). Like oil and filter changes, cheap, easy insurance.
    John Lewenauer - Newsletter Editor - Regional Director
    1961 190 SL
    Click here to EMAIL me

  5. #5


    I just realized that I said "DOT 5". I checked the bottle, I use Castrol GTX "racing" DOT 4 that I buy at Pep boys. Hence, I think that the brown discoloration is as John stated, the hydroscopic effect and having rust form in the system. One thing I don't have anymore is the metal triangular sheeting that covered the area where you can find the master cylinder. So I wonder if the fluid in the cylinder is getting rusty, I think I was lazy 20 years ago and just left it off. The master outside is covered in rust, but still works perfectly. My brake pedal is not spongy, and the car doesn't pull on braking, but I must agree with John in that brakes are not something to wait on to fail. Thanks for the quick responses and your help. Frank

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