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Red Herring · 20-07-20, 08:53 AM

The design of a caliper generally introduces fluid in the centre or towards the top of the caliper, and the bleed nipple is always at the top. This means if you don't drain them before flushing you invariably leave an amount of old fluid in the bottom half of the caliper, and given this is the very part of the brake system where the fluid is working at it's hardest this doesn't seem logical. This isn't such a problem if you replace the fluid regularly as you say you do as the new fluid will very rapidly absorb the excess water vapour from any old fluid still in the system and perform accordingly, it just means it will have a shorter life than if you had drained it completely.....

The same is not true of forks however. Fork oil is not hydroscopic like brake fluid. Water vapour will mix with it and form an emulsion (anyone seen how milky engine oil goes when a head gasket goes...?) and if you are in the habit of replacing the fluid in your forks by sucking it out from above you absolutely don't get the fluid from the bottom of the forks out (the design won't let you) and this is exactly where the contaminated fluid will be. I suppose if you go for a decent ride immediately before doing it you might be in with half a chance of getting a percentage of the crud as unlike brake systems forks do circulate the oil around them, but it doesn't take long for it to sink back to the bottom again where you will only remove it with a strip down.

If you doubt me get two small clear containers and put some brake fluid in one, and fork oil in the other. Now add a little (little!) water and give them a good shake. The brake fluid should remain fairly clear, maybe a shade or two darker but it will still be clear and if you leave it overnight it won't change. The fork oil will go all cloudy and in the morning you will see that it has started to separate again.

Changing either fluid regularly is good practice and as you say if you make it easy then it's more likely to get done, and if easy is good enough how can you argue with that.

20-07-20, 08:53 AM	#31
Red Herring Member Mega Poster Join Date: Dec 2007 Posts: 2,708	Re: no pressure In rear brakes The design of a caliper generally introduces fluid in the centre or towards the top of the caliper, and the bleed nipple is always at the top. This means if you don't drain them before flushing you invariably leave an amount of old fluid in the bottom half of the caliper, and given this is the very part of the brake system where the fluid is working at it's hardest this doesn't seem logical. This isn't such a problem if you replace the fluid regularly as you say you do as the new fluid will very rapidly absorb the excess water vapour from any old fluid still in the system and perform accordingly, it just means it will have a shorter life than if you had drained it completely..... The same is not true of forks however. Fork oil is not hydroscopic like brake fluid. Water vapour will mix with it and form an emulsion (anyone seen how milky engine oil goes when a head gasket goes...?) and if you are in the habit of replacing the fluid in your forks by sucking it out from above you absolutely don't get the fluid from the bottom of the forks out (the design won't let you) and this is exactly where the contaminated fluid will be. I suppose if you go for a decent ride immediately before doing it you might be in with half a chance of getting a percentage of the crud as unlike brake systems forks do circulate the oil around them, but it doesn't take long for it to sink back to the bottom again where you will only remove it with a strip down. If you doubt me get two small clear containers and put some brake fluid in one, and fork oil in the other. Now add a little (little!) water and give them a good shake. The brake fluid should remain fairly clear, maybe a shade or two darker but it will still be clear and if you leave it overnight it won't change. The fork oil will go all cloudy and in the morning you will see that it has started to separate again. Changing either fluid regularly is good practice and as you say if you make it easy then it's more likely to get done, and if easy is good enough how can you argue with that.