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  1. #1
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    How much fluid is in my brakes?

    Just about to collect a 2nd hand bike purchased on ebay and the brakes (formula b4s) are listed as 'needing attention'.

    One of the levers is leaking and both need 'a bit of tlc'

    Now reservoir replacement kits are available for not much cash, and a bleed kit is on order. I was also looking at replacing the original dot4 fluid with dot 5.1.

    How much will I need? (50ml - 500ml )

    Also - is there a trick to entirely replacing the fluid? Do I need to flush it out and if so what with? Will traces of water be more detrimental that traces of Dot 4 in the new system. And ultimately how do I ensure I don't lock more air in the system than is probably there already!).

    Or should I cut my losses and just bleed it with 2 syringes and Dot 4

  2. #2
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    i can't answer all of your question but:

    just get a small bottle from the autoparts store.

    don't flush with water, just bleed twice if you're really worried about it.

  3. #3
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    DOT 4 and 5.1 are 100% compatible so it doesnt matter...

    as for bleeding .. unless its got bleeder screws (like my M755's) then the syringe is your only option...

    as said above, just flush a few times thru and youll be good to go...

  4. #4
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    Cool - I was worried about the compatibility and whether or not you could mix them like different weight oils (and end up with DOT 4.6 )

    Presumably to completely change the fluid I just keep the brake lever pulled to the bar (to open the circuit) and squeeze the bottom syringe (caliper) while 'sucking' with the top syringe (lever) and fresh DOT 5.1 should fill up the system from the bottom while old DOT 4 exits from the top?

    And back to the title question - roughly how much fluid will I need to do this front and rear (having never bled brakes before) All I know is that the bleed kits come with 50ml - presumably I'm going to need more than this for this operation (I'm guessing that auto stores come in 1 litre+ bottles)

  5. #5
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    Also just done a quick general google search (rather than mtb forum search) and discovered that DOT 5 is a no-no! (How confusing )

    So DOT 3 (obsolete), DOT 4 and DOT 5.1 can all be mixed (as they are all glycol based) but DOT 5 cannot (as it is entirely different - silicone based)

    DOT 4 has lower boiling point than 5.1, but is less susceptible to water absorption.

    DOT 5 is very susceptible to water absorption and should therefore be changed frequently as it is more likely to corrode systems. It is also very susceptible to aeration and should be sat for an hour before introducing to your system as even the ride home from the store will cause it to look like a fizzy drink! Other than the fact it won't strip paint - I can't see why they even invented the stuff.

    Anyway - I only include the above in case any noob like me is reading and goes to the auto store and buys 'any old' brake fluid - Harley Davidson uses DOT 5 apparently, and introducing that to my Avids/Formulas could spell disaster

  6. #6
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    Good work on the research, you learned something today. As for fluid amount, generally brake fluid at auto parts stores comes in pints, quarts and gallons. You should be fine with a pint, you can sometimes do an entire car with a pint of fluid.

  7. #7
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    150ml should easily flush and bleed two brakes. Unless there's a huge master cylinder, each brake will typically is unlikely to contain more than 30-40ml of fluid.
    .
    .


    What luck for rulers, that men do not think - Adolf Hitler

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