Problems bleeding rear brakes

[brad28b]

Hi Guys

I got a new pitbike from DHZ and the rear brakes had an issue, so they sent me a new rear brake kit which I've just installed. It came with chinese fluid already in it so I bled it out and replaced it with something better, but after 10 bleeds I still can't get any feeling in the brakes.

I can actually see what I think the problem is, its a small air bubble in the master cylinder reservoir. I can actually see it through the clear glass panel. But for the life of me I cannot get the bubble out. Like I said I've bled it like 10 times, tried tapping on the reservoir to try to get it to go to the top but no matter what I do its stuck in there.

I've attached a pic - the bubble is right at the top of the glass panel. Any tips on how to get the air bubble out cause its driving me nuts!

Cheers

EDIT: Forgot to mention, I have followed all the guides on here including the youtube video, and have tried many different methods, but still no luck

 

[pbk]

it doesnt sound like that is your problem

sounds like the the piston is jammed

 

[Cactus Jack]

Unbolt the parts off the bike and move 'em around while bleeding 'em to get the air bubbles out ...

You want the slave to be higher than the master so that the air bubbles travel upwards towards the bleed nipple ...

Make sure you use a spacer between the disc pads to simulate the thickness of the disc ..,

 

[brad28b]

Thanks for your replies guys

Im not exactly sure what I did different this time, but I just pulled the whole kit off, turned the master cyclinder over on an angle to try and get the bubble out, but the bubble is still there - however, my brakes now seem to be working perfectly. I think I just fluked something, I've seriously been at this for about 8 hours now. Anyway for now they feel fine. Something I noticed this time when I was bleeding them is that when I'd crack the bleed nipple, with the lever already pressed down, as I cracked the nipple I felt the lever go down further and much more fluid came through on each press. Its 2:30am so I can't take it for a ride but just rolling around the garage feels good. But yeah that bubble is still there in the res, do you reckon it'll give me grief before long?

 

[Cactus Jack]

Yeah , when the slave is lower than the master and the line as when bolted on a bike it's harder to bleed air out of them ...

A bubble in the sight glass should be able to be removed by tilting the sight glass facing down towards the rear at an angle (line outlet at the extreme top)... then it'll move up to underneath the fluid re-filler cap ...

There must have been air in the line or in the slave cylinder ... The reason why brakes feel "spongey" is because air in the system is compressing like a spring and stopping the fluid from acting like a solid to push the piston ... Bubbles want to go to the top of any liquid ... so you want the master to be lower than the slave cylinder ... and the bleed nipple to be at the highest point with the line straightened and at an upwards angle ... Air will not travel downhill in a fluid ...

The worst thing anyone can do is rapidly pump the master cylinder ... all that they're doing is frothing the fluid up like coca-cola ... then later on the air reverts back to a single bubble and the brakes feel spongey again ...

To bleed brakes you open the bleed nipple , push the plunger into the master to push the fluid thru (like a syringe) ... then hold the plunger compressed while the bleed nipple is tightened up before allowing the plunger to pull back out ... If the plunger is allowed to move out before the bleed nipple is closed ... air gets sucked back in thru the nipple and you are essentially wasting your time ...

When the bleed nipple is tightened while the plunger is compressed ... new air bubble free fluid is pulled from the masters reservoir , ready to be pumped thru the line when the plunger is pulled back out ... Obviously when the master , line and slave are all filled with pure brake fluid you get maximum hydraulic push force on the slave piston/s and a solid pedal feel ... (Always make sure the master reservoir is topped up at all times to avoid air getting sucked into the system while bleeding ... otherwise you'll end up chasing your tail)

As I stated ... air at the top of the reservoir of a master cylinder is OK ... It's finely dispersed air bubbles and air in the line and slave cylinder that are the enemies ...

 

[brad28b]

I think what got it working for me was when I took the master cylinder off I laid it on the ground for a minute while the caliper was still bolted on, so that made the caliper higher than the master cylinder, possibly forcing air in the line to the caliper, which then came out when I bled it.

I wish I listened to my mate. He told me before I started "make sure you elevate the back wheel, just hoist it up so that its higher than the master cylinder". Which is the same thing you guys are saying. Although I couldn't get it hoisted easily so I just tried to do it on flat ground. And the back wheel is a bitch to get back on when you take it off, so I was trying to avoid unbolting the caliper. I think if I had done that initially it would have worked first time and saved me 8 hrs lol. Oh well, I know for next time.

 

[brad28b]

Unfortunately i'm still having some problems. Driving me nuts. The new brakes are working however the pads are rubbing on the disc quite badly. I can still push the bike in neutral, but you can feel a lot of resistance. If I put it up on the center stand and spin the back wheel, It only spins about 1/4 of a turn before it stops. The disc isn't bent or warped.

I can see that the piston is releasing fine - it is all the way back in.

In fact its the inside pad thats rubbing on the disc which makes me think the problem might be wheel alignment.

If I loosen off the axle nut a little bit, it creates a very small gap (0.5mm if that) between the inside pad and the disc, and the problem is fixed and the rubbing is gone and I can spin the wheel freely for 5 or 6 turns with no resistance. But as soon as I tighten the axle nut back up, the rubbing starts again.

I was thinking maybe im tightening up the axle nut too tight, but I don't think so. I just tighten it until the back of the bike lifts a bit off the ground a bit. Haven't got a torque wrench, but I don't think thats the problem.

Anything else I could check?

I've attached two pics, one with the axle nut fully tightened where you can see the inside pad rubbing on the disc, and also a pic with the axle nut loosened off a bit, which fixes the rubbing but im not confident enough to ride it without the axle nut being nice and tight.

Basically, it feels like either the rotor should be about 1-2 mm to the right of where it is, or, the caliper needs to be about 1-2mm to the left of where it is. With the wheel nut loosened up a little, it all aligns perfectly, but tightened, its out.

The first pic is with the axle nut loosened off a bit, and the second pic is with the axle nut full tightened up - see the inside pad rubbing?

 

[brad28b]

By the way I definately got the wheels spacers on the right sides, I took particular attention when I took them off and made sure they went back on the correct sides.

But I just had a thought, maybe the factory put them on wrong....Maybe I should try swapping them, because they are of slightly different sizes

 

[brad28b]

* double post *

 

[Cactus Jack]

Yes the longer one would go on the sprocket side ... That'd move the disc over to the right a lil' ... Otherwise you'd have to fit a washer between the caliper and mounting bracket to move it over to the left ... The caliper should really "float" to find its own happy spot ... but maybe yours is at the end of the sliders ? ...

 

[brad28b]

I checked the wheel spacers, they were definately right. The wheel wouldn't go on if I tried to swap them to the opposite sides.

Also just tried putting in a washer on the right of the caliper in the hope it would push the caliper over to the left about 1mm, but all tightened up it doesn't seem to have made a difference. Still feel a lot of resistance when trying to spin the wheel.

Im starting to wonder if it is actually the disc rubbing on the pads, or if it might be a wheel bearing problem. Its hard to tell. Its brand new and I haven't greased them or the axle or anything. Would it make sense that if the problem was the wheel bearing, it would spin fine when the axle nut was loose, but I would get resistance when tightening it?

 

[rumble]

Im starting to wonder if it is actually the disc rubbing on the pads, or if it might be a wheel bearing problem. Its hard to tell. Its brand new and I haven't greased them or the axle or anything. Would it make sense that if the problem was the wheel bearing, it would spin fine when the axle nut was loose, but I would get resistance when tightening it?

It might be the disc rubbing on the pad as you tighten the axle. take the caliper off to check that.

the pads will always be close to the rotor, and they are designed to be self centering (within reason).

I'd go for a ride and see if it fixes itself.

 

[hman]

make sure the pads are seated in the caliper correctly.

 

[brad28b]

Thanks for all your suggestions but none of them have worked.

I took the wheel off and put it back on again just because im running out of ideas. Now I've got no feeling at all in the brakes again. This is really starting to give me the shits, I dont think I will ever buy a DHZ bike again. 30 hours I've spent so far trying to fix a brand new bike so I can ride it. I never had anywhere near this amount of troubles with my Yammi

 

[TDR]

mate I'm not being rude but don't blame dhz for your bad maintenance
and if something has gone wrong just think it could a be a factory fault so the easiest way to fix that is either ring dhz and say look this n this has happened how can you help me but don't ring if you have already ridden the bike many times before this problem occurred

so if i was you i would buy a new brake assembly and get it done with because the amount of time you spend saying I'm not going to buy from dhz or comparing you Yamaha to a dhz is the amount of time you could be either trying to find the problem or buy a new part


Andrew

 

[TDR]

sorry for the double post since it's what you say a "new" bike id be ringing dhz and saying I've got the bike brand new and this has happened but be nice about it.
Depending on how you go about it is depending on how dhz go about it

 

[brad28b]

Andrew how can "my bad maintenance" be the cause, its a brand new bike? If you read the thread, you would see that I have already had DHZ send me a brand new brake assembly but that has not fixed the problem. I've been very nice and polite to DHZ and they have been to me also.

 

[brad28b]

Just to add to that, I don't want to give the wrong impression - ive actually been very impressed with DHZ's service, they have tried all they can to help me, have sent me a new brake kit etc, but yeah...it didn't help.

I guess coming from a Yammi im used to things just working. Not having a dig there, just telling the truth. I didn't really have to do much at all to it so I guess im just getting a bit frustrated. Anyway thanks for your help

 

[TDR]

ok look mate I'm sorry i appolagise but have you ever thought the disk could be bent i had the same problem and i got a new disk and fixed that but I'm not sure maybe your rim has a factory buckle that's causing the disk to rub which is a possibility and again I'm very sorry for not reading the whole thread i was trying to point out something BUT I FAILED

please forgive my laziness for not reading the whole thread LOL
Regards Andrew good luck with the brake and your bike!

 

[brad28b]

Its all good mate, no worries.

Yeah, im starting to think that either the disc or the wheel might be bent. I kind of ruled that out cause its all brand new, but I guess it could still be bent.

I would have thought though that if it was bent/warped, it would only rub in spots. But it rubs all the way around.

Im going to have a muck around with again tonight and see if I can work it out