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Hope Mini piston not retracting
#1
As I couldn't get any further with my pivot bearing replacement on the Stumpy, I took a look at Mrs Abzz Hardrock which runs Hope "Mini" Hydraulic brakes
They had started to drag a bit even when cold, so I promised to look at them for her

At first glance (see pic)one of the pistons is quite a lot further out than the other (which still sits flush in the caliper) so I wipped the pads out and tried to push the offending piston back into the caliper......... but it wouldn't budge, even with quite a lot of force from a screwdriver  Undecided
resim

so at first I thought I had a seized piston but when I operate the lever both the pistons move inwards at the same time and retract to their original position Undecided I gave the piston a clean with brake cleaner and that didnt help so I have now put a bit of dot 4 brake fluid around the piston to try an lube it, but I am note hopeful of a result  (Hope-ful, did you see what I did there   ;D:Smile)
Any tips or suggestions or is splitting the caliper and cleaning the piston the only solution (in which case I will need a seal kit?) Undecided
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#2
Try levering it in with the end of a 10mm spanner.
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#3
"Buzz" Wrote:Try levering it in with the end of a 10mm spanner.

I have tried a considerable amount of force on it but it doesnt seem to work, I have also tried forcing back with the blead nipple undone and also with the reservoir cap off in case there was too much fluid behind the piston but with no luck  Sad
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#4
Are you forcing it back in flat ? Try leaving the pads in and pushing the left piston in by wedging a screwdriver between the right piston and the right pad and pushing the right pad into the left hand pad
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#5
"TooYoungToKnow" Wrote:Are you forcing it back in flat ? Try leaving the pads in and pushing the left piston in by wedging a screwdriver between the right piston and the right pad and pushing the right pad into the left hand pad

yup Steve done that, had a flat piece of plate in there and pushed against a pad in the other side with a flatblade screwdriver.. no luck  Sad
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#6
Are the pads free in the caliper body?
Looking at the picture take the left pad out & try to clamp the right piston in place with a G clamp or similar.
Then operate break caewfully!

It could be that the left piston is stuck in so won't allow the right one to retract - hydraulic lock.
Taking the reservoir cap off will only work properly if you pull the lever too.
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#7
Sorted this on Saturday
I purchased a new Hope seal kit (£2.99) "just in case" and new set of pads (£12). Then took the plunge and drained the system by pumping the brake lever with the bleed valve open. Then I took the caliper off and stripped it down.
Hardest job was to get the pistons out of each half of the caliper. Did this by closing the bleed valve and then holding the nossle of my track pump against the hole that take the fluid behind the piston with one hand, while holding the unit on the bench in a piece of rag with the other. A few sharp pumps and "pop" out they come WARNING! if doing theis remember to cover the pistons with the rag as they come out with quite a force and you could be left with a piston size hole in your forehead!!
Cleaned everything with brake cleaner and the sides of the piston with one of those "scotchbright" type plastic dishwashing pads and lubricated the seals with a ultra thin smear of brake fluid (you are meant to use silicone grease but I didnt have any). Reassembled the unit with the new pads, reattached and bleed the caliper. Now working great!  
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#8
nice work, there's a certain pride to be taken in making broken stuff work... Especially if it's cheap too.
Keep it foolish...
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