Hope M4 stuck piston

[Battersby]

A "new" bike turned up yesterday with a hope m4 front brake. One of the pistons on the drive side doesn't move at all, the one below it moved once but once pushed in doesn't move. The "non drive side" pistons work perfectly. I've never serviced a brake before but I believe hope still sells the pistons. What can I do to get the old ones out

 

[Battersby]

OK, can get it to move now and I've put a bit of dot fluid on and pushed it back in a few times but it will only come out when the other is held back.

 

[Twozaskars]

OK, can get it to move now and I've put a bit of dot fluid on and pushed it back in a few times but it will only come out when the other is held back.

Possibly corrosion in the calliper behind the seal or the seal has swollen. It’s got to be rectified so just strip it completely and go from there.

 

[dyna-ti]

As above. New seals, and why not, new pistons. Pistons are about 6 squid a pop, and inc seals its going to be about £20 or a bit less.

Which M4 is it ? is it the black calipers with the gold bore caps ?
Exploded diagram for that model - Gives you the part numbers for buying replacements
https://www.hopetech.com/_repository/1/documents/2004Mono4-caliperexploded.pdf
Or the earlier model
https://www.hopetech.com/_repository/1/documents/2002M4Caliperexploded.pdf
When it comes down to actually doing the service, post up another thread and I and others can walk you through it.

 

[novocaine]

hold up before you strip them, have you bled them first?

 

[Battersby]

hold up before you strip them, have you bled them first?

No, my hose won't fit over the valve. Ordered a new one.

 

[novocaine]

No, my hose won't fit over the valve. Ordered a new one.

bleed them first, if there is air in the caliper it will do what you are seeing.

it may not be and it might be that the square seal has become stiff or expanded, but I'd start with the easier option first.

 

[dyna-ti]

No, my hose won't fit over the valve. Ordered a new one.

Are you using their bleed kit. or at the very least the Hope bleed nipple adaptor ? Theres 2 sizes.

Small, does the modern bleed nipple, large does the older type.
It's certainly a lot easier than trying to force a length of plastic hose over it, which inevitably falls off
I get my hose from the pet suppliers, its the same size so the fish tank air thingy is the best one and effectively costs pennies.

I cant remember when the smaller nipple came in(maybe Tech 3 or tech 2/evo) But the larger does C2/O2 and the mono's.
https://www.sjscycles.co.uk/tools/hope-bleed-kit-nipple-adapter-large-silver-httblnms/
At only £3.30, you'd be better getting both sizes. Simply labeled Small and Large.

 

[Battersby]

OK, brakes bled. Zero air in them. Still same issue. I'll try grease if not I'll have a go at replacing the pistons. Annoying as I asked the seller did everything work as it should... Maybe it was sat up for a bit before selling.

 

[dyna-ti]

I'll try grease

DO NOT USE GREASE

Grease of any type is the wrong thing, and i keep hearing about copaslip which also is the wrong thing for bike brakes, but its become kind of an urban myth.

The proper stuff to lube the piston sides is silicon lube,and in particular 'Hunters'
Hunters silicon lube is the one hope use and highly recommend. Its really cheap and a small bottle will literally last decades.
https://www.biketart.com/products/hope-silicon-lube?variant=40809527214259