newbie front brake problem

gus · Post by **gus** » Sat Apr 27, 2013 8:26 am

Hi
Try tapping the lever whilst moving it a bit. Do this quite fast with the bars turned to one side on the side stand. It really does sound like little bits of air trapped in the high point of the mc/banjo bolt. Try the elastic band over night. If still no good I would then rebleed the whole system . Do you have a mitivac vacuum bleeding tool or similar? It is the best way to bleed as it pulls fluid through the system. What span adjustment do you have the lever on? Put it on maximum when bleeding using the lever to pump fluid and fine bleed.
Hope you sort it out
Gus

pauls · Post by **pauls** » Sat Apr 27, 2013 9:43 am

ok cheers trying elastic band now leave it till tomorrow and see what happens cheers again

Dai wiskers · Post by **Dai wiskers** » Sat Apr 27, 2013 11:05 am

As gus has said tapping the lever or jiggling it is the best way if you squeeze it you will force the tiny bubbles down the hose remember that these are tiny bubbles

Corvus · Post by **Corvus** » Sat Apr 27, 2013 7:37 pm

Regarding the technique of putting an elastic band around the lever overnight. I'm not a mechanic so I'm asking questions here, but I've always been intrigued as to what exactly happens in this circumstance?

What it will do, as far as I can see, is move the master cyl cup seal in front of the vent hole, preventing any air from getting out into the reservoir. Until the lever is released at least. It will obviously put the liquid trapped ahead of the cup seal under pressure. Any trapped air will be under exactly the same pressure.

After the initial lever movement the pressure will be a static pressure, with no fluid movement. As far as I can see the air will still rise even though it is under pressure, because the pressure will be equal to the fluid pressure and air is lighter than brake fluid. But will the air be persuaded to rise any easier than it would if left under atmospheric pressure, with the vent hole open?

What exactly happens?

gus · Post by **gus** » Sat Apr 27, 2013 7:52 pm

Hi
For whatever reason the lever feels firmer.
It's magic I tell thee....

Gus

Corvus · Post by **Corvus** » Sun Apr 28, 2013 7:40 am

[quote="gus"

"It's magic I tell thee...."

There's a lot of it about. Especially where motorcycles are concerned.

slparry · Post by **slparry** » Sun Apr 28, 2013 7:43 am

gus wrote:Hi
For whatever reason the lever feels firmer.
It's magic I tell thee....

Gus

It's the workshop fairies aligning the molecules of the brake fluid overnight when the lights are out.

Merecat · Post by **Merecat** » Sun Apr 28, 2013 11:38 am

Ya cannae change the laws of physics Jim.

You put the system under pressure, the fluid is incompressible (is that a real word) but the air is not, so the fluid will force its way in to the spaces occupied by the rogue air. This air then gravitates to the highest point in the system which will be just behind the master cylinder piston. and exits in one splurge (techie term) when the lever is relaxed.

Or thats what I think

Mick

pauls · Post by **pauls** » Sun Apr 28, 2013 12:08 pm

ok update I tried the elastic band overnight took off this morn brake rock hard, road to my mates bout 15 miles everything spot on, went out for a spin with him soon as we hit the twisty roads lost the brake again? soon as you hit the flat it pumps its self up again and is rock hard? cheers

gus · Post by **gus** » Sun Apr 28, 2013 12:18 pm

Hi
OK. It sounds like the the pistons/ pads are getting knocked in. Check discs /pads and wheels for true running. Also check for excessive movement in the wheel. Could be wheel bearings.
Gus

Corvus · Post by **Corvus** » Sun Apr 28, 2013 3:53 pm

Merecat wrote:Ya cannae change the laws of physics Jim.

You put the system under pressure, the fluid is incompressible (is that a real word) but the air is not, so the fluid will force its way in to the spaces occupied by the rogue air. This air then gravitates to the highest point in the system which will be just behind the master cylinder piston. and exits in one splurge (techie term) when the lever is relaxed.

Or thats what I think

Mick

That makes a lot of sense to me.

But it presumably can't always displace air in a persistent high spot. Or there wouldn't be such a thing as air in the system. This bubble of air will stay lodged and will be compressed when pressure is applied. Any high spots should have a means of venting.

Is a banjo fitting accepted as a means of venting? The banjo fitting appears to be a brilliant way of guaranteed trapping of air, albeit a very small amount. Some people seem to frown upon venting via the banjo fitting. Others recommend it.

Cheers.

pauls · Post by **pauls** » Sun Apr 28, 2013 5:10 pm

just checked wheels/discs everything spot on its only done 18k? like a say I have had at least 60 bikes I am a cbt instructor as well and have never had a brake like it? cheers

ianbcr · Post by **ianbcr** » Sun Apr 28, 2013 6:33 pm

dont want to rub salt into the wounds,but when i put braided lines on mine,this was the easiest brake blead i ever did.but i would go for a top banjo bleed,as this seems popular on the tl sites.

Merecat · Post by **Merecat** » Sun Apr 28, 2013 6:58 pm

Corvus wrote:
That makes a lot of sense to me.

But it presumably can't always displace air in a persistent high spot. Or there wouldn't be such a thing as air in the system. This bubble of air will stay lodged and will be compressed when pressure is applied. Any high spots should have a means of venting.

Is a banjo fitting accepted as a means of venting? The banjo fitting appears to be a brilliant way of guaranteed trapping of air, albeit a very small amount. Some people seem to frown upon venting via the banjo fitting. Others recommend it.

Cheers.

Pressurised fluid will always displace air in the system. As Gus says turn the bars to ensure the fluid res is the highest point to allow the air to bubble out through there and not pool at the banjos.

Back to the original problem. It does sound like for some reason the pistons in at least one of the calipers are retracting. All I can suggest is next time it happens, stop without using the front and have a look to see if the pads are way off the disks and then take it from there.
Are the pads sliding ok on the pins? If the pins are crapped up the pistons will force the pads on to the disk and then spring back over a short time giving the symptoms you have, Just a guess, but something to check.
Has this suddenly started? Did you change something that caused it?
Its only simple mechanics the cure will be simple too.
Just need to logically work through it.

Good luck

Mick

pauls · Post by **pauls** » Sun Apr 28, 2013 7:55 pm

have tried everything but still the same? must have bled every bike I have had and never had a problem, never had to do banjo bleed but have done it and still the same? it was like it when I got bike bmw mechanic did a service for me and changed fluid but still the same,he then said try braided hoses I did but still the same tried a different master cylinder still the same? its just odd? cheers