Rear disc brake bleed

A

andre

Senior member
Guys
Stunning autumn (fall) weather here. Ideal for riding if we were allowed...so I'm making something good of this darn lockdown in NZ by doing some well needed Laverda maintenance.

Rear disc on my Motodd has a conventional 70's master cylinder, 280mm disc and a smaller Brembo caliper (off a D#$%ti or something).

Despite some pumping through of fresh fluid, with the bleed screw shut off I see lots of bubbles coming up in the reservoir on the out stroke of the m/cyl piston.
Any wise words why it might do this? Very frustrating. Tempts me to fit a later model master cylinder.

Thanks, Andre
 
Air has to go somewhere Andre. Have you bled it properly?

Pump pedal up, hold down pedal while opening the bleed screw, watch all the crud, fluid and air come out, close bleed screw (still with pedal down) and repeat the procedure until you have pure fluid coming out.
 
chrisk said:
Air has to go somewhere Andre. Have you bled it properly?

Pump pedal up, hold down pedal while opening the bleed screw, watch all the crud, fluid and air come out, close bleed screw (still with pedal down) let the pedal go and repeat the procedure until you have pure fluid coming out.
Click to expand...

... and make sure your bleed nipple on the caliper is the high point.
 
I've pumped plenty of fluid through following ye prescribed procedure.
Bleed nipple is on high side of caliper. I'm wondering if the 'bubbles' coming up out of reservoir are brake fluid not air?

I'll check brake line routing. It is a up and down route.

Cheers
 
A lot of it depends on how quickly you do the job...generally bubbles of air will track to the highest point...Try and eliminate the highs from the line...bubbles like to congregate at high points..can you disconnect the caliper and make it the lowest point with a straight uphill run to the m/c..  Another idea is to push the fluid in at the caliper and send everything uphill.bubbles like uphill..
 
Up from MC and down to calliper? There?s the problem, pesky air will be caught at the high point. Take calliper off, jam some flat bar, thicker than the rotor, between the pads
and bleed again while your offsider holds the calliper up in the air.
 
Doesn't really matter where what is, as long as the air can escape.

I actually do it with the M/C on top.  Gentle actuation of the piston nudges air out bit by bit.  The M/C can easily be topped up as fluid level drops.  The opening at the caliper bleed nipples is often too small to allow air to flow on its own accord, the caliper pistons can be pressed inwards slightly to get things moving.

With the handlebars in the right position and providing the calipers are suitably mounted, the front system can be bled without any additional aids at all.  Might take a tad longer than with a vacuum set-up, but quite foolproof, and especially handy by the roadside.

piet
 
Andre said:
Despite some pumping through of fresh fluid, with the bleed screw shut off I see lots of bubbles coming up in the reservoir on the out stroke of the m/cyl piston.
Click to expand...

That's as it should be, Andr?!  As others have said, the air is moving slowly up the system to arrive at the highest point, the master cylinder.

Sometimes you just have to be patient, especially when the air has been broken up into lots of small bubbles - normally because you've been pumping the lever too vigorously, too quickly  :-X.  Slowly and gently, gently and slowly ...

Andre said:
I'm wondering if the 'bubbles' coming up out of reservoir are brake fluid not air?
Click to expand...

;).  No, the fluid itself doesn't form a bubble.  The only thing that can form a bubble in the fluid is air.

Andre said:
I'll check brake line routing. It is a up and down route.
Click to expand...

Yes, do try and eliminate any high points in the brake line where the air can stick.  Straight lines is what you want.  Sometimes it's necessary to remove part (or even all of the brake line  :o) to achieve this.

Remember, air rises:  any air that is in the fluid above the caliper is trying to get out at the TOP.  You can't push it down the line to exit thru the bleed nipple on the caliper.  That would be to defy the laws of physics  :P
 
bubbles often are sucked in between the bleeder-nipple and the seat/the thread of the nipple. Don't expect just fluid if you drain the system, especially if you have an underpressure system to suck the fluid out... The only way to make sure there is no air anymore is as Piet described: press the pistons back into the housing as much as possible (bleeder nipple closed, calliper down and M/C up, brake line connecting to the M/C must be the highest point of the M/C when you press the pistons back in).

after that, pump until you got pressure and repeat.

Any time I have to take the M/C (front or rear) off and need to bleed the system afterwards, I first take the callipers off of the brake disk, pump a bit more fluid into the calliper(s), take of the master and work on it, mount it back, fill it as much as possible and then simply press the pistons of the callipers back in. all air goes back up to the M/C and after some pumping, you only need to top up the fluid to the right level, job done. If the fluid is old, you can also first empty the reservoir before you top up, pump a bit until you have pressure, normal bleeding of some fluid and do the same again after you are sure only fresh fluid is in the calliper. Easy.
 
Ventodue said:
Remember, air rises:  any air that is in the fluid above the caliper is trying to get out at the TOP.  You can't push it down the line to exit thru the bleed nipple on the caliper.  That would be to defy the laws of physics  :P
Click to expand...

You can, actually.  The brake lines are small enough on the inside to literally push a bubble of air through them downwards with the brake fluid.  But this has to happen quite fast and steadily, at least faster than the rate at which the bubble would rise.  The action of the brake lever doesn't push enough fluid for this, air bubbles just move to and fro during the pumping, with a bare minimum of fluid passing "past" the bubbles, giving the impression that something is actually HAPPENING, when it's not...

piet
 
The bubbles were rising in the master cylinder, so nothing to do with open bleed nipples.
I put grease on the bleed nipple threads as this inhibits air being sucked in when the nipples are opened.
The kit I use is a Mityvac, which sucks the fluid at the bleed nipple end.
Not a fan of kits that pressurise the reservoir and push fluid through.
I had a seal go in one of those kits at the master cylinder cap (Eezibleed) and the fluid soaked the bike.
 
I use the MityVac as well.  I'm looking to buy a device that supplies a constant vaccum though, leaves both hands free.

Air sucked past the bleed nipples is no big deal.  Enters through the thread and is sucked out by the vacuum device, no problem.

piet
 
sfcpiet said:
..  I'm looking to buy a device that supplies a constant vaccum though, leaves both hands free.
Click to expand...
Invested in an 'Air-Boy' air brake bleeder (124143100) a while back.
Not cheap but have used it with success many times. And works well enough using my small air compressor.
Mark
AirBoy-brake_bleeder-124143100_02.JPG


 
Yes, something like that, Mark.  As I've broken a couple in my career, I need something a little sturdier, with more metal in it.  Fuckin' gravity... ::)

piet
 
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