Brake binding

[avinitboys]

I am having issues with my front brakes on my recently finished project bike. I've only been able to do a few short trips but have already noticed a problem with the front brakes beginning to bind after a very short time. At first I thought it was a sticking piston in a caliper despite both being rebuild with all seals and the worst of the pistons replaced. I swapped these out with a known good pair but the problem was still there. All the while I'm thinking possible warped disc but I just cant see any issues there when I spin the wheel. I then rebuilt the master cylinder in the vain hope that it would cure it but no joy there either so now I am convinced the discs are a little out. When cold I dont feel much resistance with the bike up on the stand or pushing it about but after riding for a very short time the lever goes very hard, the brakes start to bind like hell and the discs are almost blue.
Is it the discs? Could it be a fork alignment issue. Both looked straight enough to me when they were stripped and on the bench. Is there a sequence to set the forks in the yolks? e.g. with or without the front wheel in place.
Casting my mind back a few months I remember it being a little difficult to get the wheel spindle in. Have I bent something possibly or was it just that the fork alignment was out and this is whats causing the problem.
Is it worth loosening everything off again including steering head bearings etc to be absolutely certain everything is in line perfectly.
If all else fails then a nice set of wavy discs it is then!
Any thoughts guys?

[Old Painless]

I would look at the pistons first, pull the calipers off one at a time and make sure the pistons are returning into the caliper body easily when pushed, did you lube the seals when you fitted them ?

[avinitboys]

Yeah checked that already and have swapped out both calipers with a set I know are fine from my other bike. Same problem.
Went for another ride today after rebuilding the master cylinder and same thing still but when I got back after 5 minutes cooling down the lever feel returned to normal and the binding had almost stopped. Im puzzled

[Old Painless]

If both calipers are good it must be the disc or contaminated pads ?

[avinitboys]

Yeah that was another thing I looked at and also swapped out the pads to make sure. Also forgot to mention that today when I got back it seemed that both discs were very hot, with one being hotter than the other. When I say hot I mean, almost blue and absolutely no way you could lay a finger on them without being burnt. Thats why I thought master cylinder but maybe it is just that both discs are slightly warped. Would even a neglible amount of warping give that kind of heat build up? And why does the binding ease of when the discs cool? Didn't think that expansion could account for it but maybe I'm wrong.
At the end of the day, I bought the bike as a bundle of bits that had been sitting for 7 years in a garage so who knows whats occured during that time. The wheels were still on a rolling frame though so I don't think they were ever laid flat.
May go out again later and try to do a few miles without using the front brake at all and see what happens.

[hunter]

How much fluid is in the reservoir,if it's overfull it will pressurize the system and cause the brakes to bind.It starts with heat transfer from discs to pad to fluid.

[avinitboys]

I'll have another look but I'm sure I'm not overfilling it. Is it worth running with it at a low level to rule that one out do you think? Am I correct in thinking that the diaphragm in the resovoir is just there for expansion purposes? What I'm getting at is.........could something as simple as that be at fault?
Also should say that today this heat build up happened very quickly, probably less than a mile and I only touched the front brake a few times from less than 30mph. I suppose that if the system is pressurising then binding will happen very quickly right? Would the pressure then subside somehow after cooling as it did today?

[avinitboys]

Right, just been down the road for half a mile and instant binding again, almost to the point at which the bike didn't want to go forward. Managed to coax it back at low speed. Phew!
Emptied the resovoir as low as I dared go so thats ruled out. I just don't get this one at all. One side was still way hotter than the other so I'll treble check that caliper again. Maybe my eyes are playing up and the discs are more f****d than I think they are. Suppose I should take the discs off and check them properly.
Thanks for the input guys.

[Deviant400]

try swapping front wheel and discs from another bike

[Cammo]

Would even a neglible amount of warping give that kind of heat build up?

I doubt it, pulsating would be felt through the brake lever long before they get that hot.

Have you checked the bores of the calipers and master cylinedr? Rebuilding them with new seals won't help if they're scored.

Does the master cylinder piston spring return action feel ok when there's no fluid in it?


I've had m/c woes before (no fun) in the end I gave up on rebuilding them and brought a brand new one.