Rear brake dragging

Hey all, my rear brake is driving my nuts!

84 GPz1100 unitrack

After about 10 mins of normal riding the rear disc heats up to such a point the rear wheel won't turn by hand. Once left to cool I can continue until it heats up again and i have to stop.
Starting out on the ride the brake operates fine - no dragging until it heats up.

Here's what I've replaced so far:
Brand new OEM master cylinder
New OEM master cylinder piston
New stainless steel brake line
New reservoir supply pipe
Rebuilt caliper fully cleaned inside with new seals and piston
Brand new EBC pads - even tried thinner worn pads (made no difference)
Dot 4 fluid fully bled

What I've checked:
Both master cylinder feed holes (supply and smaller return) - clear
Plastic 90 degree connector - clear
Adjusted the brake lever rod in and out - makes no difference to the outcome.
Caliper piston moves freely and returns OK when its cool
Disc doesn't have runout

While its binding on the disc when hot if I crack the bleed nipple it will release the caliper and goes back to normal function.

I've bled the whole system repeatedly and can't find any air bubbles.

Its behaving like the master cylinder feeds are blocked and not allowing the piston to return slightly, then drags and heats up - but both are clear.

Has anyone had anything similar? I'm getting to the point now where I'm thinking off putting on old parts just to see if it makes a difference.

Cheers,

wrote:

While its binding on the disc when hot if I crack the bleed nipple it will release the caliper and goes back to normal function.

Its behaving like the master cylinder feeds are blocked and not allowing the piston to return slightly, then drags and heats up - but both are clear.

 

As you said, it sounds like the fluid return hole/path is blocked.  And if it restores normal function when you open the bleeder, it doesn't seem like it could be some fault in the caliper.  It's not supposed to build up pressure in the line.

Have you tried opening the reservoir cap when it's in this state?  Maybe pressure is building up there?  Doesn't seem likely, but it's something else to try.

Are you sure the MC push rod/piston is fully released to the end stop so both MC ports are open?
Try to "bleed" the rear caliper with the MC banjo bolt. If the wheel than breaks free your problem is the MC.

Yeah, I’m leaning towards the MC myself. Will try cracking its banjo next time it happens.
I’m also thinking maybe i have been sent a MC piston that is too long. Won’t know until i strip it again and compare with an old one.

Another possibility is that the flexible line is deteriorating inside. I had this problem in a car many years ago. I could bleed the brake system like normal but there was enough blockage in the bad line to keep the brake pads from retracting and the brakes would drag.

Thanks mate, good suggestion. It's got a new stainless hose so that bits probably OK.

hi woodyyale ,i had the same problem om one of mine ,if you have a pattern piston seal it may be just gripping the piston a bit to tight and make sure the groves where it sits are spotless of grud  

Well I think I've found the problem!

Check out the pic of the new MC piston I was (wrongly) supplied with against one from an old MC i had lying around. At least 5mm longer.

I guess it wasn't clearing the relief ports in the MC and was building up pressure, clamping the caliper on and not allowing it to release.

I'll know for definite once I have a new correct one installed but it does seem likely to be the problem.

Thanks guys for all the pointers, it really helped.
 

Was this the actual problem?  I'm having the same issue on an 81 KZ1100 but I threw out the old plunger so cannot compare to the new one I got from Brakecrafters.

Yes it was. The brakecrafters one was too long for the mc by about 3mm, so it was blocking one of the return valves and locking pressure and dragging the brake after about 10 mins of riding.

i ended up managing to source an oem piston and seal from Vintage kawasaki in canada. Couldn’t find one anywhere else.

once fitted that sorted the problem straight away.

i would try and locate an oem one, if posible, then at least you can rule that out as a problem if its still the same.

 

Thanks for the reply. I just got an oem replacement and I'm also switching hoses.  Fingers crossed!!