Front brakes seizing

Hi Guys, had a bit of bad luck as of late with the GRiSO, first last month I had the tappets fail - now rollered... at my cost...
Today - on the way to work the bloody front brakes slowly started to seize, then pretty much locked up altogether at an intersection.. not cool
once they cooled down they loosened up so off I went.
on the way home they done it again - checked once home and both calipers very hot to touch, and front wheel wouldnt spin if raised.- so both appear to be seizing.
I took the right side caliper off and checked it out, pistons go in and out etc with nothing apparent wrong... checked the lever was seated correctly etc and looked ok.. took it for a ride again and within 1km of on off braking it done it again - heating up part of the problem?? I am lost.

any ideas lads??

The lever needs free play where it pushes on the back of the master cylinder piston.
Some bikes have a pin (rod) between the two. I made a rod for my Cali Stone years ago to remove all the free play. Guess what happened.

As you ride, normal pad drag creates a little heat - and expansion.
For the pads/pistons to push back, the fluid has to go somewhere. Back up to the m/c.

Next possibility is dirt in the master cylinder. Only needs a speck to block the tiny return bleed hole.

Once you've checked the free play, I'd suggest stripping and cleaning the calipers. It's a bit of a pain but not a huge job.

thanks for the advice guys - I think the lever has plenty of free play as the pin moves at least 5mm before it appears to hit the plunger(?)
think I will flush the MC and clean the calipers - cover both those bases first and report back!

cheers

5mm? That's a lot. Maybe the m/c piston isn't returning to home.

yep Id say about 5mm - see video. I just cleaned calipers, flushed and bled lines, not test riden yet - need to wait until this evening.
whats everyone elses brake lever play like?

I reckon fresh fluid should fix things. Brake fluid is hygroscopic (sp?) meaning it absorbs water over time, especially in an already opened container. Heat plus water = expansion, which then just increases the pressure in your brake lines. Happened to me once, after a few solid stops and a hill, bike shuddered to a stop, but would travel OK after cooling off. At first I thought it had seized up.

Errrr? No. For the system to pressurise as described the priming hole from the reservoir has to be blocked off.

Pete

Mine was basically the same as you described Rouleur, before it locked up the first time it was shuddering too. Unfortunately it's now chucking it down outside so don't really want to see if my front will lock up again after the fluid change... hopefully its dry tomorrow.
BTW I flushed with methylated spirits before running about 500ml of new fluid through. read somewhere that was okay - so if there was any sort of blockage hopefully its now gone.

Shuddering 'might' indicate a warped disc, or a seized caliper piston.

It was fast shuddering, more like extremely harsh vibrating, and only at slow speed comming to a stop so don't think it's a warped disc. Seized piston maybe, but didn't notice prior to cleaning them and all were working fine after the scrub. Makes sense though.

disc shudder??? try cleaning the disc bobbins.
easy And cheap!!
fixed my 'warped' disc!

ok, so, just got back from a test ride after the following
1. brake caliper clean - no full disassembly, just a thorough scrub with brakecleaner and a toothbrush. all pistons move freely
2. brake fluid flush and bleed - flushed with meths and then ran 500mls of new fluid through the entire system.
........no dice....... again pressure slowly built up - I used the rear brake more and just used the front a little to see if it got worse with more use, which it did.
whats next? MC rebuild? I my gut tells me it's something to do with the piston, see free play in the vid above.. doesn't look to be returning home properly but cant recall what it was like before

Without touching the lever drop the hose off the master and get a thin probe of some sort, (A clean screwdriver will be fine.) and poke it into the end of the master until you can feel it touch the piston. Now push on it and see if the piston moves back further.

If it does then something is causing the piston to bind and the MC might need a rebuild or even sleeving. If it doesn't move back any further then it is most likely just a blocked primer hole. They compressed air from both the reservoir side and the delivery side.

Pete

Thanks Pete, piston didn't move so might be the primer hole, how does one go about cleaning that - do I need to pull it down to pieces or is there another way?

Look into the reservoir and at the bottom you should be able to see two holes, a larger one and a smaller one closer to the wet end. It's the smaller one that you need to ensure is unblocked so expanding fluid can return to the reservoir. You can use a thin pin to poke through it or compressed air to blow it out.

Pete

dammit - it was me arrrggghhhh  
took the lever off on Sunday to give it a clean and the pin must have been incorrectly seated when reassembled... has folded over the end of the clip stopping the piston from returning home properly
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That'll do it!