Brake Bleeding

[l3ddy]

Evening all, I'm having a lot of trouble bleeding the front brake on my 125 race bike, something that should be so simple! Basically I have fitted a new braided front line (single disc set up), added new fluid and gone to bleed it, brake worked fine before just had a scuff on the line I wasn't happy with.

When I try bleeding the first time I crack the nipple I get a few bubbles and fluid push out, then I can close the nipple, pump the lever as many times as I want and just get nothing the next time I crack the nipple? Not got any lever pressure or anything. I have tried the following;

Standard bleeding procedure

Pushing fluid through from both ends with a syringe- both times the pressure built up in the syringe and no fluid could enter the system?

Left lever cable tied to bar for a day or two

Bled with the caliper off, hung it upside down, literally tried every way of forcing air out from it.

Stripped the master cylinder down and it all looks good in there too no signs of leaks etc.

Somebody please point out the obvious I am missing or give me something else to try before I set it on fire and hit it with hammers?

Thanks

[dansp1]

Open the reservoir lid and just gently paddle the lever in and out, let it spring out quickly and don't pull it all the way in, you should see bubbles coming back up the line into the reservoir ( air is funny like that, it wants to go up not down)

It can help to tap the caliper and brake line from time to time in order to release trapped air, you can often get a perfect lever like this, but will sometimes need to bleed from the nipple on the caliper or remove the caliper and rotate it.

[lorenzo]

I've had a situation in the past when air has been trapped behind the pistons in the caliper; try pushing the pads back and wedge them open and give it a try.

[gurninman]

Try bleeding at the master cylinder banjo first.once no air appears there , bleed the caliper end

[l3ddy]

Thanks for the replies, should have stated the pads/pistons are already all the way back and I'm getting no movement on them. There is no bleed nipple on the master cylinder, I have cracked the banjo bolt to get fluid out a little when I first started. I have tried tapping the caliper, jiggling the line etc all to no avail.

I just don't understand why if I went to it now the first time I cracked the nipple I'd get a load of bubbles and fluid out but after that nothing? It's like I need to leave it 24 hours to get anything out the next time I try? I've just bled a clutch for my brother and that was a piece of piss, so I'm pretty sure my bleeding procedure is all ok.

[bikecat]

blocked bleed nipple or blocked hole in the caliper maybe..

[the_doctor_46]

ive had it sometimes where it does this, try it with the caliper pistons extended slightly.

also, check the small fluid return hole in m/c isn't blocked (sometimes its hiding under a removable baffle)

[lorenzo]

Have a cup of tea, sit back and look at everything.

This should be a sticky at the top of every workshop thread.

[l3ddy]

Thanks again for the replies. I spent yesterday having another look, much tea was drunk and still nothing!

Stripped the whole lot down and blew the caliper through, cleaned bleed nipple and then tightened up all connections etc. Tried pumping the lever in different ways and several different caliper positions for bleeding. I've about given up on it to be honest, there must be a problem in either the master cylinder on the caliper.

Looks like I'll have to order a couple of rebuild kits? It's frustrating because a single disc set up must be the simplest brake to bleed and because it worked before replacing the line. If it isn't broke don't fix it I guess!

[porter_jamie]

If you tried pushing fluid in from the bleeder and it wont go in maybe the bleed port in the master cylinder is blocked. Have you fitted an aftermarket lever? Try again with the lever removed

[l3ddy]

I have fitted a new lever into the original master cylinder yes, didn't think to change that if I'm honest. What is the bleed port? There is no bleed nipple up top, just the one on the caliper. I fitted all new copper washers after yesterday's strip down and I'm as confident as I can be that they are tight enough.

I'd pay good money right now to know what is wrong, but at the same time if it's a case of having some patience and a good technique I'd much rather save that money!

Again thanks for the replies and advice, I may have another go this afternoon if I can work out a plan of attack.

[Wildbill]

Failure in ability bleed a brake system after an overhaul when it was working previously is invariably down to master cylinder seals.

The seals operate over a short distance when the system is working correctly...........

once the system requires bleeding the old seals have to operate over the full stroke of the cylinder and either stretch,leak or fold and wont pressurise the system.

On inspection often nothing appears amiss with the seals or cylinder bore.

New seals installed the correct way around and the master bled separately is my advise.

If you remove the line at the caliper and bleed it to there using your thumb and forefinger over the union and then zip tie the lever in and block off the union with copper washers and a bolt, the next throw of the brake lever should result in a rock hard system if the master is correct.

On rubber lines versus the braided ... just lock the rubber line off with a brake hose clamp tool.

[dansp1]

the bleed port is the hole in the bottom of tha master cylinder that allows fluid to top up the system, it is open when the brakes are off, or should be, lots of cheap pattern levers are made so badly that they effectively hold the brake on a little thus covering the port so fluid can not go into the system.

try the original lever, if it all works you can dress a little off the new lever or put a decent one on.

[krazyk]

Bit of a rave from the grave but I had an old Yam. in which wouldnt bleed out. Having tried the usual I discovered a pin head sized air bubble in the m/c in front of the piston,

when pumping the leaver it moved into the cylinder. I used a very fine wire to disturb the bubble and it moved up the wire and out.

Be carefull to only touch the bubble when doing this. Hth.

[Wildbill]

^Tapping the m/c body gently with a screwdriver handle usually yields the same results.

[l3ddy]

Bit of an update, tried again yesterday, I removed the lever and the cap from the master cylinder. Pumping the plunger with my thumb rather than using the lever resulted in loads and loads of bubbles rising into the fluid reservoir.

At first I thought great I've sorted it, but after half an hour of doing this and a numb thumb I came to the conclusion it must be drawing air in from somewhere? I've triple checked all connections and there's no leaks anywhere.

I think I'm going to just buy a complete front brake set up from a breaker and then rebuild this one with new seals etc when I have time. I'm desperate to get this working to use on my acu licence day so without knowing what is actually wrong Its abit if a gamble buying a rebuild kit for both ends.

Again thanks for all the replies and advice, really thought I'd found the problem yesterday!

[dansp1]

how much did you top up the system with when you started getting bubbles?

[l3ddy]

I pumped the bubbles through for around 10 mins and then emptied the reservoir because it was full of tiny bits of muck etc, filled with new fluid and carried on for a further 15 mins? It never really went down in level just got muckier in colour.

There was always fluid in the feed pipe too whilst I emptied and refilled so no danger of letting air in when I did that. Do you think it's worth spending some more time trying to pump the air out? Pistons still havnt moved in the caliper yet.

[dansp1]

if the level is not going down it is not filling the system...try opening the bleed nipple and see if fluid comes out, don't pump it just let gravity do it's thing, if you start getting fluid out lock the nipple up and keep paddling the lever , if the level doesn't go down and you are getting bubbles it does sound like you have a problem

[Superdunc]

Is the new brake line you've fitted a pre made one with crimped on ends, or a the type with screw on compression fittings and brass olives?

[andy tims]

I fitted Goodrich speed bleeders to my track bike. Not sure if they would cure the OP's specific problem, but they do make the whole process much easier.

[l3ddy]

It's a pre made crimped one dunc, banjo bolt into the master cylinder and then the line screws itself into the caliper body at the other end. I have just ordered a complete front brake set up from a breaker on eBay, not going to find me my problem but if it allows me to use the bike then it'll do for now!

[Superdunc]

I have seen braided lines with the screw on ends, where during assembly the inner liner has ended up with a flap of plastic swarf ,created when the end fitting was inserted, that has done a very good impression of a one way valve.

Worth blowing an air line both ways down the brake line just in case something like this has occurred.