how to bleed brakes/replace fluid?

[motorbyclist]

today i had to put a new reservoir/master cylinder (single unit) on a dirtbike and did it "my way", and had it done in no time with no bubbles and little spillage.

meanwhile my mate was doing his bike after replacing the banjo copper washers and did it "his way" and after 15 minutes gave up.

i know my brother does it another way again!

how do you guys do it? i'm pretty sure my way is wrong, because it's too easy.

my way:

summary: use big syringe to squirt fluid from caliper up to reservoir - bubbles flow out naturally, different colour fluid to old stuff shows when you're done
detailed method with explanation below;
get a big syringe (approx 60-90ml, must be more than the surprisingly small volume of your brake line - standard equipment on a farm, pays to flush/rinse with fluid first) and an inch of clear, soft plastic tube (maybe 4mm id, fits and seals onto nipple on caliper perfectly). put the plastic tube on the end of syringe.
first i duct tape an ice-cream container so it's hanging under the reservoir, then move the bars/bike/reservoir so the brake line is all uphill - logic being the bubbles will flow out easily.
open the reservoir lid.
open the valve on the nipple on the caliper just to free her up and give a little squirt to clean out any grit in nipple that is already as clean as possible.
Fill syringe from bottle of fresh brake fluid, put the tube/syringe onto the nipple and open the valve - don't bother sucking the bubbles out.
make sure the syringe is held so fresh bubbles are not going to go into the brake line and then squeeze.
the brake fluid flows back up the brake line, carrying the bubbles with it, through the open valves on the master cylinder and into the reservoir, which then overflows into the ice-cream container. if you can watch it (i use a mirror) you can see any bubbles coming out, and if you're clever you're using a different colour brake fluid to your old stuff so when the new colour overflows the reservoir you know you're sweet.
close nipple valve, pull syringe off, then use syringe to suck excess from reservoir. put lid on reservoir, test brakes, and if all's good clean up. i recommend squirting everything with detergent and hosing off as i've run an experiment and can confidently say that brake fluid is an effective paint stripper

i'm concerned that if there's any crap in my nipple that i don't clean out (partly why i give a bit of a squirt before starting) i'll simply pump it up into my master cylinder valves - but i hasn't happened yet.

biggest spill i had was accidentally squirting the ceiling with my syringe and having fluid rain down on my other bikes


his way:

put long, clear tube on nipple valve and run into a receiver bottle/pan. open lid on reservoir and top up with fresh fluid.
now alternating between brake lever and nipple valve, manually pump the fluid through the system top down, against the natural flow of the bubbles, and watch bubbles flow through the tube into the receiver. remember to top up reservoir.

he pumped like this for 15 minutes and never saw an end to the bubbles, so gave up. apparently his way was "what the internet said" - he probably should have used a different colour fluid to see when he had flushed through


brother's way:

identical setup to my way, but instead of squirting the syringe he sucks the fluid down the system, and has to top up the reservoir. he had a lot of trouble with bubbles, wasted about 250ml of fluid and eventually gave up & got dad to do it - and i dunno how dad did it but it worked



SO, how do you guys do it?

i like my way as the bubbles run straight out, but i don't know what the risk of contamination is, and suspect my mate or my brother had the correct method executed poorly. although, if you keep a rubber cap on the caliper nipple i think my way is reasonably safe.... then again, the first thing i always do upon opening a reservoir is drop the lid, inside face down of course, onto some dust/grit/sand on the floor, so i probably get shit in the reservoir anyway

[imdying]

A syringe isn't necessary, just a piece of hose, a jar, and the correct technique... I've posted it a few times if you search around.

[hellnback]

Mate, your way sounds very messy!!

[Max Preload]

I just bleed the master cylinder first into a rag then quickly hook up the lines.

A guy I went to school with had his master cylinder off for a holiday tart up of his CB250RS. When he reassembled the brakes, he didn't know to bleed the master cylinder and rode around for 2 weeks with only rear brakes because his old man told him the front would come back in a couple of weeks and to just keep topping the fluid off.

[vifferman]

Your method sounds good apart from two things: overflowing the reservoir with brake fluid is a bit messy and you'll end up with some mixing of the old/new fluid in the reservoir. But for changing the fluid with minimal bleeding, it's very good! If the old fluid isn't very old, I guess it doesn't matter, and if it was, then I guess you could completely empty the lines/cylinder first, run a bit of new stuff through, then use your method, but without overflowing the M/C
You're right to say that the 'normal' methods of manual bleeding make it hard to get rid of all the bubbles - a lot of tapping of banjos and lines is required to persuade the bubbles to move, especially if there are junctions where they can hide. (I replaced the lines on the VTR100 for that very reason)
A vacuum bleeder works well, but most of us don't have one of those.

[Reckless]

HIS way is the common way. But there is a technique to it.

Firstly don't shag with your brakes if you don't know what you are doing! You kinda need them to work every time. Seriously! if there is air left in them you can pump them up in the garage and they will seem fine, then leave them for an hour you'll have nothing on the first squeeze. I'd hate this to happen to you when you are riding.

I think the biggest mistake You made was not staying to help dad and learn from someone who seems to know! DUH!

[motorbyclist]

HIS way is the common way. But there is a technique to it.

Firstly don't shag with your brakes if you don't know what you are doing! You kinda need them to work every time. Seriously! if there is air left in them you can pump them up in the garage and they will seem fine, then leave them for an hour you'll have nothing on the first squeeze. I'd hate this to happen to you when you are riding.

I think the biggest mistake You made was not staying to help dad and learn from someone who seems to know! DUH!

thanks, and yeah yesterday i wished i had bothered to learn back when dad did it

yeah i know what it's like to suddenly have front brakes fail right when you need them, so be sure i will be testing to see if my way really did work - just it never occurred to post here till after i did it


and it wasn't very messy! well, by my standards anyway