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I have probably posted this before - but thought I had solved it with a good clean and relate of all the pistons. But it is up to it's old tricks - generally after a fast downhill - and braking.
And then it doesn't unlock - or at ;east not consistently. Throwing cold water on it doesn't help - there is no evidence that the rotor is getting cooked - so not over heating the fluid.
Any suggestions - I can only think that the five year old pistons are finally toast - and it's time for service at Hope. Any other ideas?
Did you re-lube the pistons with silicon lube? I had an E4 calliper that would lock on after use, and ended up stripping and replacing all the piston seals, and then lubed the pistons with the recommended silicon lube as they were refitted. Not had a problem since.
And the other bike has recently had the E4 pistons re-lubed as a precaution (didn't replace any seals, as that's a messy and fiddly job). Used a 'bleed block' that allows a single piston to be squeezed out at a time. Then attacked the piston with lube and cotton buds. Force back in, out, more lube, force back in, out, more lube. Repeat 8 times for both ends...
This is the stuff: https://www.biketart.com/products/hope-silicon-lube?variant=40809527214259
(Other purveyors of said lube also available)
And something like this: https://www.ebay.co.uk/itm/266238193288?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=7lnhsUW6TXm&sssrc=2047675&ssuid=hEpFFh9CTji&widget_ver=artemis&media=COPY
to squeeze one piston at a time.
When was the last full bleed and what fluid did you use?
As the fluid ages and absorbs more water, the boiling point can drop enough to cause what you describe despite the brake getting no hotter than before.
How worn are the pads and what rotor thickness are you using? V4 is designed to take the thicker vented rotor. They will work with a standard thickness rotor but when the pads get too worn, it's as if the pistons can come out at a bit of a wonk and get jammed.
Pistons don't slide through the seals when you brake. The seals flex in and out, that's how the pads return. They only slide through slightly as the pads wear, to take up the slack. So I don't think lubing will help.
It's possible that your mounts are wonky or the caliper's misaligned on the mount, which would cause the pressure on the pistons to be uneven and that could cause them to lock. I've seen stuck pistons before, but only one side at a time.
Just change the seals and hoy the new hybrid pistons in.
What @onzadog says.
It's more common once the pistons/seals dry out and a bit dirt builds up on the pistons stopping things from moving as freely.
Working the pistons out, running a bit brake fluid around them, and pushing them back in will help in the short term, but a full strip down to give the pistons/seals a good clean (or replace if you want), and rebuild with a bit fresh silicon grease is a longer term fix.
BITD when Hopes pistons had springs on them, returning was no problem. Im not saying they could cobble together the same system on todays brakes, but it sure would solve the problem.
BITD when Hopes pistons had springs on them, returning was no problem. Im not saying they could cobble together the same system on todays brakes, but it sure would solve the problem.
They did use to have piston return springs but that was only on the closed system brakes. You can't do that on open system brakes or the lever travel would increase as pads wear.
What @onzadog says.
+1
Twice I have let the pads run a little low on E4 and X2 callipers and had the brake lock on. As the V4 is intended to take wider discs I’d imagine it is an even more likely issue on those
Done the re-lubing bit - with that very lube - in an out 3 times. But they can be a bastard to much back in
Bled with DOT 5.1 about two weeks ago.
They could be wonky but the howl like a banshee when wet or hot - but setting them up via various methods makes F all difference.
Ror
Rotor is a cheap Disco brakes one
Brake now on its way to Hope to sort out, Now rocking an SLX rear and saint front. I have never used shimano brakes before!