Se7enBTCC Posted February 15 Share Posted February 15 This will be a stupid question I'm sure, how do I change the brake fluid when the existing fluid is quite new and looks the same, i.e. do I empty the system of all fluid first, if not how do I tell when the new stuff is coming through? Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted February 15 Member Share Posted February 15 There's always been enough colour difference on mine to spot the transition. Have you directly compared old and new? I wouldn't deliberately introduce air as a marker. I think that leaves relying on volume: typically 500 to 750 mL. Jonathan PS: Is there a named law on "stupid questions"? IMHO those who suspect that they might be are usually wrong... ; - ) 1 Link to comment Share on other sites More sharing options...
Drumster Posted February 15 Share Posted February 15 The only stupid question is the one you don’t ask…. 1 Link to comment Share on other sites More sharing options...
PlastererPete Posted February 15 Share Posted February 15 How old is the fluid you are replacing? Personally I would get a brake fluid tester and see how bad it is/isn't before going through the hassle of changing it. I wouldn't take the testers answer as gospel but its certainly a good indicator. Link to comment Share on other sites More sharing options...
aerobod - near CYYC Posted February 15 Share Posted February 15 If the brakes have been used in any "sporting" way, the fluid near the calipers when bleeding will be darker than the new fluid. If the reservoir is emptied with a syringe or baster before bleeding and filled with new fluid, 125ml bled at each caliper (500ml total new fluid) will flush the system completely. 1 Link to comment Share on other sites More sharing options...
Beagler Posted February 15 Share Posted February 15 (edited) Average mileage for everyday car use is 12000 per year. Usually recommended brake fluid change is every 2 years. The brake fluid lack of darkening colour is irrelevant as brake fluid is hygroscopic so as more moisture is absorbed this can vaporise and form bubbles although the fluid in the reservoir may look ok. Change the fluid every 2 yrs for low mileage road use or earlier if heavy track use. Edited February 15 by Beagler Link to comment Share on other sites More sharing options...
Area Representative bluenose Posted February 15 Area Representative Share Posted February 15 (edited) Used to use Super Blue until it was banned 🙄 Edited February 15 by bluenose Link to comment Share on other sites More sharing options...
Leadership Team SLR No.77 Posted February 16 Leadership Team Share Posted February 16 Get a tester. Brake fluid only needs changing when it is contaminated, usually too much water in it. We’ve been indoctrinated by car dealers to believe it needs changing every two years so they can make more money, unless you’re doing a lot of track work and overheating the fluid this really isn’t necessary. My tin-top had the original fluid checked at 3 years / 60,000+ miles and it was assessed to be fine! It really depends on use. Link to comment Share on other sites More sharing options...
Graham Hutton Posted February 16 Share Posted February 16 I’m sure that’s correct but changing it this winter did give me another couple of undisturbed hours in the garage and that was well worth it! I might add it to my list of winter jobs every year!!!! 2 Link to comment Share on other sites More sharing options...
Miker7 Posted February 16 Share Posted February 16 I'd second the brake fluid tester. Got one from Aldi or Lidl for about £6. Tested my new to me Caterham and it was less than 1% moisture so good (hopefully) for at least a year or two & saved me a job ....... after i'd bought the replacement fluid Link to comment Share on other sites More sharing options...
Beagler Posted February 16 Share Posted February 16 1 hour ago, Miker7 said: I'd second the brake fluid tester. Got one from Aldi or Lidl for about £6. Tested my new to me Caterham and it was less than 1% moisture so good (hopefully) for at least a year or two & saved me a job ....... after i'd bought the replacement fluid Where do take the test sample ? Link to comment Share on other sites More sharing options...
Roadsport06 Posted February 16 Share Posted February 16 Is testing the brake fluid in the reservoir a reliable check? That assumes that the brake fluid/water ratio is homogeneous throughout. Is that the case? I had read somewhere that the water migrates to the lower point, usually the calipers. Link to comment Share on other sites More sharing options...
Beagler Posted February 16 Share Posted February 16 No, its doesn't get pressurised as its above the actual master cylinder. When a fluid is compressed it produces water so that moisture will be in the brake fluid beyond the piston and seals as its under pressure when the brake pedal is depressed. You would need to sample the fluid from the caliper bleed nipples in which case you might as well bleed the system. Link to comment Share on other sites More sharing options...
John Vine Posted February 16 Share Posted February 16 I can see that water content is important, but are there not other factors to consider when assessing the need for a fluid change -- such as the effectiveness of the corrosion inhibitors? JV Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted February 16 Member Share Posted February 16 2 minutes ago, Beagler said: When a fluid is compressed it produces water so that moisture will be in the brake fluid beyond the piston and seals as its under pressure when the brake pedal is depressed. My emboldening. I don't understand the effect that you are describing. More, please. Thanks Jonathan Link to comment Share on other sites More sharing options...
SM25T Posted February 16 Share Posted February 16 Brake fluid absorbs moisture from the air. Water is heavier than hydraulic oil, so it falls to the lowest part, which is the calipers. Link to comment Share on other sites More sharing options...
Area Representative Golf Juliet Tango Posted February 16 Area Representative Share Posted February 16 2 minutes ago, SM25T said: Brake fluid absorbs moisture from the air. Water is heavier than hydraulic oil, so it falls to the lowest part, which is the calipers. Easy really, although at what speed is the next controversial question. I leave these matters to a professional, being well aware of my level of incompetence. Link to comment Share on other sites More sharing options...
aerobod - near CYYC Posted February 16 Share Posted February 16 With 1% water content (assuming sampled from near calipers) a high boiling point high performance fluid will have degraded to that of an average fluid, losing about 50C of it’s boiling point: https://epicbleedsolutions.com/blogs/faq/what-is-meant-by-wet-and-dry-boiling-points-of-brake-fluid. For this reason, any track that I have attended in Canada requires fluid to be no more than 6 months old when used from a sealed container, when self-certifying the tech inspection of the car. 1 Link to comment Share on other sites More sharing options...
John Vine Posted February 16 Share Posted February 16 22 minutes ago, SM25T said: Brake fluid absorbs moisture from the air. Water is heavier than hydraulic oil, so it falls to the lowest part, which is the calipers. That's not how it works, Ian -- at least, not for the more common glycol-based brake fluids. They are water-miscible, so there is no separate "water component" to settle out. The moisture from the atmosphere is absorbed and disperses evenly. The danger is that this moisture will vaporise at the calipers due to heat, and it's the vapour that causes the unwanted sponginess (or worse). Link to comment Share on other sites More sharing options...
Area Representative Geoff Brown Posted February 16 Area Representative Share Posted February 16 Two years for a Seven as recommended. Yes, test the fluid but as stated earlier the increased water content lies away from the master cylinder. I just change mine on spec, (a) I know the brake hydraulic system will perform however hard I rely on it (b) I can go & hide in the man cave for an hour or so making a mess (c) SWMBO knows where I am ! If one has a relatively new car coming up to it's first brake fluid change I would in all honesty suggest carrying the process out. It is not just the water content being removed but I have assisted local members with a fluid change to find small solid particles ejected from the bleed nipples !! In fact my own home build back in 2013 revealed solid particles ending up in the bottom of the glass jar. Don't forget the clutch fluid as well. Link to comment Share on other sites More sharing options...
John Vine Posted February 16 Share Posted February 16 13 minutes ago, Geoff Brown said: In fact my own home build back in 2013 revealed solid particles ending up in the bottom of the glass jar. Alarming, Geoff! I wonder if that was a consequence of reduced inhibitor performance? JV Link to comment Share on other sites More sharing options...
Beagler Posted February 16 Share Posted February 16 3 hours ago, Jonathan Kay said: My emboldening. I don't understand the effect that you are describing. More, please. Thanks Jonathan Absorbs moisture from micro porous hoses or atmosphere. Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted February 16 Member Share Posted February 16 3 minutes ago, Beagler said: Absorbs moisture from micro porous hoses or atmosphere. Thanks Yes, but it was the effect of compression... 4 hours ago, Beagler said: When a fluid is compressed it produces water so that moisture will be in the brake fluid beyond the piston and seals as its under pressure when the brake pedal is depressed. Jonathan Link to comment Share on other sites More sharing options...
John Vine Posted February 16 Share Posted February 16 2 hours ago, Beagler said: Absorbs moisture from micro porous hoses or atmosphere. Like JK, I don't understand what you meant by "When a fluid is compressed it produces water...". Can you elucidate? JV Link to comment Share on other sites More sharing options...
7 wonders of the world Posted February 16 Share Posted February 16 The biggest source is via the cap and this is dependant upon what type of seal and vent arrangement you have, most tintops simply vent to atmosphere where as other components manufacturers - Wilwood, AP Race etc employ a sealed expansion bladder arrangement primarily for fluid containment, however the fluid can only access moisture from a small volume of air - there is no vent to atmosphere on the fluid side. The biggest effect you can have is not only bleeding correctly but purging all the old fluid form the calipers, especially if you intend track driving, the rears have a tendency to hold a pocket of old fluid due the caliper being mounted at 90 degrees to its original design. Some makes of fronts (and rears) offer 4 bleed nipples, you can therefore drain the calipers of old fluid and debris via the lower ones prior to bleeding the upper ones, the std rears being a single piston design need removing to purge the old fluid and bleed. Link to comment Share on other sites More sharing options...
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