Whats the best method of replacing brake fluid:

a. drain the old so the system is basically dry and then refill

b. drain the old to the bottom of the resivour and then flush through with new



Cheers


Alex

Best option is 1st (ie completely clean it) but you'll probably need to re-bleed it afterwards quite a few times (PITA!!!)

But 2nd option is a lot easier and unless you've had a whole load of water entering the system its not needed. The fluids pretty gloopy anyway so its not as if you'll get much mixing in the almost empty resevoir... In real life the end result is so close #2 is the winner..... not worth the extra faff...

Yup, what I generally do is pump all the old stuff through, when it gets near the bottom of the reservoir then fill (or half fill - depending on reservoir size and how much fluid you want to waste) with new and pump all that through again, then bleed properly with the second top-up. Obviously you need to do an element of the "flush" with all 4 calipers, and ideally on all bleed nipples if more than one per caliper.

Don't get too hung up about changing every drop of fluid what you are really doing is getting the moisture out the system.

(1) siphon out reservoir as best you can
(2) Push caliper pistons back --- best to let fluid out through nipple. (if you have proper "rubber" brake hoses clamp each pipe as you do so.
(3) Refill refill reservoir.
(4) Pump out the caliper pistons.
(5) Optional -- bleed a couple of pumps through each nipple.

Alex, just in case you are doing a tin top with ABS be aware that you shouldn't retract the calipers by pushing them back without first releasing the bleed nipple. Not releasing the fluid pressure through the nipple can fook the ABS.
Cheers, Pewe

I would be inclined to take advantage of the situation and generously bleed each caliper through until the m/c level goes down to a safe minimum, refill with new fluid and then repeat the process until fresh fluid emerges at each caliper.

[Edited on 13-7-07 by RazMan]

Thanks for all the info folks.