Brake bleeding

G'day Cam,
The way I do it doesn't involve any stripping (of calipers that is).
Once all the fluid has been changed in the lines using the standard methods, I remove the pads and pump the brake to drive the pistons out.
Place a cable tie on the lever to stop any fluid coming back into the reservoir.
Open the bleed screw (with hose and catch bottle attached) and force the pistons back into their bores, thus forcing the fluid behind the pisons out.
Do the procedure a couple of times to flush out all the crap.

First time I did it, I couldn't believe the colour of the fluid that came out but I suppose it had been sitting there since the bike was built.

No more brake fade!!!!

Terry
 
You can either leave the calipers on the forks or remove them. I remove the calipers, take out the pads and place a tyre lever between the pistons, when the pistons are extended, I twist the lever, pushing the pistons back in.
Haven't been game enough to let the pistons so far out that they touch for the reason you mention.

Terry
 
Bleeding the brakes is such a simple thing to do. If done annually you will keep down corrosion and moisture in the lines. Also when you do have to do it you'll find that the bleed nipples aren't rusted in fast.

I know that alotta race guys change the fluid every meeting.

I also at tyre change intervals, carefully let the pistons out a tad and clean any crud away from the seal/piston area..again part of the ritual and gives me something to do while Chris is searching for the most expensive tyre for me :)

I use the same kinda Halfords gismo tube that Shaun uses although I usually get my friend to help me as it's a social event in the spring..a bit like Groundhog day.  I guess it would be better done in Autumn though so the system doesn't have moisture sitting there all winter.

joe
 
Have learnt from modern bicycle disc brake systems about bottom-to-top pressure bleeding. Any effort to push the pistons back into the calipers before bleeding will purge a lot of crap out of there.
 
I sometimes do bottom to top pressure bleeding. It's the logical way to do it because it carries any air bubbles upwards, which is the way they want to go. Also, you can crack banjo joints at high points in the system where bubbles might get caught (the banjos on ends of master cylinders on clip-on handlebars are typically the highest point because the bars are angled downwards). Since the system is held under pressure, the bubbles will be pushed out. It's very pleasing to see bubbles come out of a banjo joint until they stop and all you get is clean fluid, but it's by nature a messy business and you need to have plenty of cleaning rags handy to catch the fluid.

There are other potential mess-making situations with pressure bleeding:

Firstly, the threads on the bleed nipples won't hold fluid under pressure when you pump the stuff into the caliper. You need to take them out, put thread tape on them and screw them back in. They may still leak, but not as much as before. I've tried sealing the threads with grease, but it only works for a few seconds until the fluid forces its way past. Maybe a really thick and sticky grease would work.

Secondly you need a tight fitting hose on the barb of the bleed nipple, otherwise it can blow off under pressure and spray a fountain of fluid all over the place. A little hose clamp would help, but I never seem to have one the right size. I have tried twisting wire around the hose, but it's too bloody fiddly and inconvenient. A tight-fitting rubber hose works most of the time.

Thirdly, you need a reliable way to apply a constant pressure that's sufficient to push fluid through the system, but not so much pressure as to cause the problems mentioned above.

For my first attempt at pressure bleeding I used a large syringe, but it required a lot of effort to push the plunger in. Also, you need one hand to hold the syringe and the other hand to push the plunger in, leaving no hand free to open and close the bleed nipple. I reckon the syringe system of injecting fluid could be made to work more easily if it had some kind of jacking system to press the plunger in that could be operated with one hand.  Putting the syringe in a mastic gun should work, but I never thought about doing that until after I made system #2 below.

For pressure bleeder #2, I made a sealed canister that held a volume of brake fluid. It had a fluid outlet at the bottom with a ball valve to close it off, and it had a Schrader valve in the lid so I could pump it up with air to force the fluid through the bottom outlet. Checking the canister pressure with a tyre pressure gauge, trial and error revealed about 10 to 15 psi to be ideal. Any more than that and you'd get a fountain coming out of the master cylinder reservoir. But even with that low pressure the feeder hose would sometimes blow off the nipple (gets quite slippery when wet with fluid). That's when the compressed air system really excelled in making a mess. It would hose fluid all over the place until you could either shut off the feed valve or release the air pressure from the canister.  But if you could manage to use the pressure canister without getting yourself and the bike covered in fluid, it worked exceedingly well on even the most difficult systems to bleed, such as Guzzi's linked front and rear brakes.

I've since bought a vacuum bleeder which does the job most of the time. It sucks the fluid out of the bleed nipple so you have to make sure the master cylinder reservoir level doesn't drop too low during the process. Being under negative pressure, it eliminates the possibility of pressurised fluid squirting all over the workshop, so it's much cleaner and hardly a drop of fluid is spilled. However, since it sucks the fluid downwards through the braking system against the natural tendency of bubbles to rise, you still can get stubborn bubbles that will hang up somewhere in the system and refuse to travel downwards.

 
Dellortoman said:
Firstly, the threads on the bleed nipples won't hold fluid under pressure when you pump the stuff into the caliper. You need to take them out, put thread tape on them and screw them back in. They may still leak, but not as much as before. I've tried sealing the threads with grease, but it only works for a few seconds until the fluid forces its way past. Maybe a really thick and sticky grease would work.
Click to expand...
I was looking at various bleed nipple designs after someone posted some details of nipples that have sealant already installed on the threads to stop that problem.
I stumbled on 'Stahlbus Speed Bleeder' nipples on Demon Tweeks.
Apparently you don't have to undo the nipple at all to bleed!!!!! Just undo a fitting 1/2 turn on the upper part to release a non-return valve and then bleed away with no air ingress.
One tincy wincy issue - ?26.46 each! Might be worth it though:

http://www.stahlbus.com/info/en/service/faq-s/bleeder-valves/64-stahlbus-compared-to-other-products
http://www.demon-tweeks.co.uk/motorsport/brake-bleeders/stahlbus-speed-bleeder-valve

Lueften_Moto1_eng_2-blue-Gudea.gif


Dellortoman said:
Thirdly, you need a reliable way to apply a constant pressure that's sufficient to push fluid through the system, but not so much pressure as to cause the problems mentioned above.
Click to expand...
There is a system that uses the pressure from a spare car tyre to provide a reservoir of pressure - Gunson Eezibleed.
 
Used Eezibleed once.
Fine for as long as the seal to the m/c reservoir held.
But when it let go there was brake fluid everywhere.
Not a fan of pressurising the reservoir, prefer nowadays to use my Mityvac which sucks fluid out of the bleed nipples.
 
Grant said:
Used Eezibleed once.
Fine for as long as the seal to the m/c reservoir held.
But when it let go there was brake fluid everywhere.
Not a fan of pressurising the reservoir, prefer nowadays to use my Mityvac which sucks fluid out of the bleed nipples.
Click to expand...

I used a Mitivac, but still didn't help with upside down nipple, rear brake.
Made up a 5-6mm mount, took caliper off put caliper on mount pushed lever down and opened nipple, farted like an old mole, one more tighten of the nipple, one pump of lever, loosen nipple and fluid pissed out. Tighten nipple and refit caliper to mount .............. simples.
 
Speed-Bleeders are similar and have been around for years: pretty sure I had them on my CooperS in the early 80s.


As long as you don't over tighten them they're fine..



http://www.speedbleeder.com/install.htm
 
With you there Cosi; a jar, length of tubing and a ring spanner is all I need ::)
 
Legs said:
With you there Cosi; a jar, length of tubing and a ring spanner is all I need ::)
Click to expand...


Useful on a car if you don't have a mate about to press on the brake pedal...
 
When trying to bleed a completely new and empty brake system (as I do frequently) a vacuum bleed system makes it so much easier.
 
As I said, as just a plain owner of one bike, a jar a length of tubing and a ring spanner is all I need :D
 
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