Next Mechanical Question... Dragging Rear Brake

[Deleted account 210609001]

This is driving me insane...

The rear brake on my 650GS is dragging/binding. I have replaced the pads, the pins, the springs, I've bled it until the cows come home. The piston isn't stuck and slides back in easily enough, the caliper moves back and forth on the sliding pins with the lightest of touches.

Testing it quickly after each episode of changing something and it is fine. But as soon as I use the bike on the road it starts dragging - then it gets hot, drags more, gets hotter, drags even more. Repeat ad infinitum.

It's a single piston, sliding caliper affair. It has ABS.

Any suggestions welcomed, please.

 

[davegs]

Brakes

I know you'll think b..... but are you sure your foot is no in contact with the brake pedal, happened before many times so make sure that there is pldenty of clearance between foot and pedal.

 

[Timolgra]

The master cylinder sticking on a 650s the common cause.

I've melted a piston calliper on one and had to replace everything

 

[Dansin]

Have you checked the master cylinder for corrosion? The rod runs in a nylon bush which gets squeezed by corrosion causing the back brake to bind. My thread on sorting it here.

 

[Deleted account 210609001]

Hadn't even thought of the master cylinder - never come across it being an issue before.

I will go and check...

(Yes, I'm sure my foot isn't on the lever, but I understand your point.)

Thank you all.

 

[John Roberts]

Nah- it couldn't be- but, just in case ...

A year ago I noticed that my back brake was getting spongier and that it needed pumping to be effective. Found it was really hot. Had a proper look in the morning and took it to bits. Found that one of the wheel bearings had collapsed.

Two things:
I blamed the funny handling on the new back tyre that I had fitted the previous day, the supplier couldn't get the one I had asked for and I blamed it on the new one.

I had started to use a pressure hose to clean the bike and had, I thought, taken care not to point it at the hubs ... I then checked and found that the front bearings were also rough. Twat.

 

[Timpo]

Ensure you have free play in the brake pedal/lever.
There is a small blanking valve that shuts off the reservoir and transmits the fluid to the caliper when the lever is pressed, so if there is no free play, the fluid can not return to the reservoir when pressure on the pedal stops. So the brake stays on causing drag....

 

[Deleted account 210609001]

I've just had a preliminary inspection, and there is indeed something amiss with the master cylinder. Pushing the pedal down then releasing it leaves a lot of 'slack' in the pedal for the next push. The rod that goes from the pedal to the mc is returning, but whatever it pushes isn't sliding back down, leaving a lot of free play.

That would fit in with the symptoms and the above explanations - as far as the caliper is concerned, the brake lever isn't being released completely.

The visible rod itself is corroded underneath the rubber bush, but is still going up and down with the brake lever. Is this something I can solve with the repair kit from Motorworks? Do I need to replace the rod itself?

 

[Timolgra]

A new cylinder comes with the rod and isn't too expensive, that's really your best option by far.

 

[(RIP) Kaister]

Ensure you have free play in the brake pedal/lever.
There is a small blanking valve that shuts off the reservoir and transmits the fluid to the caliper when the lever is pressed, so if there is no free play, the fluid can not return to the reservoir when pressure on the pedal stops. So the brake stays on causing drag....

this.

 

[Dansin]

The visible rod itself is corroded underneath the rubber bush, but is still going up and down with the brake lever. Is this something I can solve with the repair kit from Motorworks? Do I need to replace the rod itself?

It's not the rod that's the problem and the Motorworks kit won't solve it. See my post above for the solution if you don't want to buy a new master cylinder.

 

[Timolgra]

It's not the rod that's the problem and the Motorworks kit won't solve it. See my post above for the solution if you don't want to buy a new master cylinder.

Good thread, I didn't know that was available

 

[John Roberts]

... A year ago I noticed that my back brake was getting spongier and that it needed pumping to be effective. Found it was really hot. Had a proper look in the morning and took it to bits. Found that one of the wheel bearings had collapsed.

I looked at this again and realised that I hadn't explained it very well. I'll try again, it could possibly be of use to somebody in the future, but I've no idea if it might apply in the case of a 650GS.

The calliper on mine ('82 RS) is fixed to the swing arm and the wheel spindle. As one of the wheel bearings had collapsed the disc was tilted and was running out-of-true in the calliper and was rubbing against the two pads which had been pushed back to the limit of their travel. As said, it could be of help so someone in the future.

Sorry.

 

[John Roberts]

Ensure you have free play in the brake pedal/lever.
There is a small blanking valve that shuts off the reservoir and transmits the fluid to the caliper when the lever is pressed, so if there is no free play, the fluid can not return to the reservoir when pressure on the pedal stops. So the brake stays on causing drag....

If crud collects behind the piston in the master cylinder that also can prevent the piston from returning all the way and opening the port.

 

[Deleted account 210609001]

It's not the rod that's the problem and the Motorworks kit won't solve it. See my post above for the solution if you don't want to buy a new master cylinder.

Thanks.

I don't want to buy another master cylinder complete for £77 quid (I don't know where you saw them for £45!) if I can fix it myself for substantially less. from you post onF650.co.uk it seems as if you didn't actually need the Motomecca kit - simply cleaning the bits sorted you out. Have I read that correctly?

But at £15 it seems to make sense to buy and fit the new parts in case any of the original bits are knackered. If they're not knackered, I'll have the old bits as spares!)

So I'll buy the Motomecca kit you identified and see what I can sort out. I'll let you know how it goes. If it doesn't solve the problem, I am completely at a loss - I'll have replaced everything by then! Time to buy a new bike, perhaps?

 

[Micky]

I've just had a preliminary inspection, and there is indeed something amiss with the master cylinder. Pushing the pedal down then releasing it leaves a lot of 'slack' in the pedal for the next push. The rod that goes from the pedal to the mc is returning, but whatever it pushes isn't sliding back down, leaving a lot of free play.

Thereby lies your answer Robin, happened to Sue's in Kyrgyzstan

The rod pushes up against the master cylinder, but that is not returning when you release the brake!

Probably corrosion inside the master cylinder, strip it down and have a look, but you're probably delaying the inevitable ....

Bite the bullet, fit a new master cylinder and ride free in the knowledge that you're OK for the next few years/few thousand miles

 

[Dansin]

Yup, I cleaned it all and reassembled. It's been fine ever since. I did use the Motorworks kit as I had already opened it but its a bit of a con at seven odd quid plus post for 2 seals.

I googled the brembo part number for the master cylinder and found it at m and p or somewhere similar for £45.

 

[Deleted account 210609001]

Yup, I cleaned it all and reassembled. It's been fine ever since. I did use the Motorworks kit as I had already opened it but its a bit of a con at seven odd quid plus post for 2 seals.

I googled the brembo part number for the master cylinder and found it at m and p or somewhere similar for £45.

Where did you find the part number, please?

 

[Dansin]

Where did you find the part number, please?

It's one of these.

 

[Giles]

Would you like me to come round and fix it for you Choir boy ??