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stuck front brakes

A

AndrewGelling

Tuned and Synch'ed
GT Famiglia
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So as part my rebuild, I added a second front rotor and second f08 caliper to my 850t. I also installed a new, appropriately sized, brembo master cylinder and custom cut spiegler lines. They bled fine, and stop the bike wonderfully. However, they are not releasing, and so the front wheel won't spin.

I have not yet had a chance to disassemble and investigate, but hope to do so this weekend. Anyone have any insight. I am sure it must be some dumb oversight of mine during assembly, but I can't think what it might be.

Thanks.
 
I suspect that it'sthe old caliper that is not releasing and not the newly installed caliper if in fact it is new. The F08s on the 850 T came with chrome plated steel pistons/puck which corrode over time and jamb in caliper. Fortunately Brembo sells the new anodized piston and seal kit for the F08.

To get the stuck piston out, I disengage the caliper from the fork then pump the master. Then clamp down the piston that is moving freely and pump out the troublesome side. Once the halves are split, then a shot of compress air quickly shoots out the remaining piston.
 
Thanks. As it had been fine last fall, I hadn't actually considered the old caliper as potentially being the problem. As for the chrome caliper pistons, I did rebuild the old f08 a few years back, and I believe I installed the new anodized pistons. But hell, that was 15 years ago, so who knows! The new caliper is not just new to me, it is actually "new" new.
 
Before taking things apart, here is a little trouble shooting. Once you have pumped up the brakes and they don't release, open one of the caliper bleeders. If fluid escapes and the wheel then rotates, then the master cylinder is not releasing pressure. This could be from a blocked orifice in the master, to even an over full reservoir. If fluid does not escape and the wheel will not turn, then investigate the calipers, starting with the older one.
 
If your new brake hose were cut to fit and hand assembled make sure that a little piece of inner hose was not left inside during the assembly. It can act as a one way check valve and keep the pressure from returning to the master cylinder.

If you pump up the pressure and the wheel is locked and you break the hose loose at the master cylinder & the wheel stays locked then the problem is between the master cylinder and the calipers. Either a hose is clogged or a caliper is locked.
 
I just wanted to say thanks. The results of the much appreciated diagnostic suggestions were ambiguous, as I had two problems! One at the top and one at the bottom. I did indeed have too much fluid in the reservoir, and the old calipers were sticking. I had indeed made the anodized piston update, but after perhaps two decades of benign neglect, they needed a little love. Not sure why I chose to overlook rebuilding them after doing everything else, but oh, well. They are now in fine shape.

Still unclear on how much fluid to put in the reservoir. Presently half way up the little round window, and the brakes are working nicely. But there are no "fill to here" hash marks. I've got the new brembo mastercylinder with the rectangular reservoir.
 
Half way up to just about 3/4 of the window should be fine. You just need to allow room for expansion. If you add fluid over time, then install new pads, you may want to remove some fluid.
 
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