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Wife could do with some new disc brakes on her city bike, was thinking of robbing these off her never used mountain bike but they need a good service.
Is it worth it? how long will pads and spares likely be around?

Is it worth it? how long will pads and spares likely be around?
Yes. Likely forever.
Yes, definitely worth it.
I got parts for fifteen year old Hope Minis without problem.
Hope pride themselves on being able to supply old parts.
They couldn't supply new hub shells for my pair of Pro2s (SS rear) - both with flange failures. There comes a point when old stuff becomes too old to be worth supporting.
The no longer do spares for 6 pots. But I have hopes of that generation and older on all my bikes.
I'd say yes, I'm rocking same vintage Mono Minis on my winter hardtail getting ridden a fair bit these past months. I rebuilt one caliper with seals beginning of year and they are doing a fine job.
Yes unless it's the really old closed system stuff. But I can't get blades for my Mini levers any more 🙁
The no longer do spares for 6 pots. But I have hopes of that generation and older on all my bikes.
dont be hamfisted and all the service parts are STD componants.
Dont burst a piston and all you need to service them are some square seals(pistons) and some lip seals(lever)
I have a lovely set of those Mono M4 brakes. Made me think I ought to service them and pop them on the FS but they're IS mounts and I suspect brackets to retrofit and align might be a problem. Or are they?
Mono M4 calipers are they?
Well the E4 pads are different but close enough they’ll fit.
So get them refurbished and they’re good until the next refurbishment ‘cos E4 pads are here for a bit.
And that’s the longest any refurbishment will ever last so it’s not a waste of money.
Failing that I’ll swap you some M6100 Shimanos for them.
Mono M4 calipers are they?
I thought the OP's were from a quick glance but could be wrong.
You can still get pads for the Mono M4 from superstar.
My brothers still got mono M4 IS version pre phenolic pistons, still going strong, several companies still doing pads for M4.
E4 pads have more material than original M4 pads so can work better, depending on the disc.
Arent m4 pads the same as one gen of Shimano anyway?