I’m using a Shimano SLX, works very good (with MountainUni external disk and now with my custom internal one).
Only thing i changed: the brake pads. Im now using Koolstop (better lifetime and brake power than the original Shimanos)
I have been looking at disc brake options, something inexpensive, mineral oil, easy to bleed/shorten lines, durable, aluminum lever:
This one is on sale, I just bought two sets, comes with the rotor (rotors are not cheap), but needs an IS adaptor (I had a couple adaptors sitting around):
Here’s an inexpensive IS adaptor for the above:
Or get it complete and spend more:
Or get it in white and be “the boss”:
Chain Reaction Cycles only has ten pairs left of the 2009 Tektro Aruiga, so order soon if you want a set. You do need a 160mm IS adaptor, but they’re cheap, most LBS should have spares and might even give them to you for free!
If you order a disc brake, keep in mind that some don’t come with a rotor, so add that cost…
The Aruiga comes with a rotor for $50 plus ~$8 shipping, add another $6 for the adaptor if you need it, still a whole lot less than $98 for a complete 2010 Aruiga, and a whole lots less than other choices.
You don’t need much brake to stop a uni, even a wimpy disc brake is stronger than a Magura on it’s best day. Disc brakes don’t get overheated like they do on bikes, so a small disc is both lighter and more durable plus it places the caliper further from the ground and your feet.
I run an inexpensive Shimano M535 160mm on my Oregon and it’s more than enough.
Also, when ordering a disc brake, look at the angle of the line as it leaves the reservoir, some angle down steeply which could complicated cable wrapping/routing.
I am so ready for disc brakes on my 29 and 26, it has been rainy and slimey here for months!
Ben, thanks for all the info you add to these threads, I for one really appreciate it.
So, when we’re looking for a disc brake, is there a consensus on whether we should be going for front or rear, or does it not really matter?
FYI, the 2010 (but not 2011) Magura Marta disk brake is compatible with the Spooner. This is the only Spooner-compatible disk brake that I know of. I’ve been running this brake as one of my disk brake options for about a year and it works well. May be hard to find, though.
Kris
For other disk brakes, a universal KH brake lever extension will be available by June.
Good to know Kris about a somewhat universal handle mod for spooners. I’ve been lopping off the nub on many hydraulic handles, and have heard of very few where this isnt required. Im suggesting Shimano, Quad Dime, Gusset and Formula RX, and Hope for those wanting brake bling.
Sorry, I meant to post that info. So you are looking for a front brake for two reason, one is that the line is shorter so you probably won’t have to cut it, two you need the front brake I.S. adaptor to fit the D’Brake from Nimbus.
More info on disc brake standards: there are two standards, the 51mm IS and the 74mm post mount. All of the current disc brake calipers have the mounting holes spaced at 74mm, which works for direct mount to some mtb forks and frames. But, for the remainder, the calipers must be reduced to a 51mm I.S. (International Standard) us a caliper mounting adaptor.
If you see a brake advertised as “post mount” and it is not shown with an adaptor, then you need to buy one as an additional cost. There are also post mount adaptors, you don’t want those, you want the I.S. adaptors for the rotor size you choose, so 160mm, 185mm, 210mm, etc…
Disc brakes use two kinds of fluid, mineral oil and DOT. Mineral oil is oily, buy wpn’t eat your paint, whereas DOT is a solvent. I prefer mineral oil, but DOT is fine, esp since DOT is what is used in motorcyle and automobile brakes.
Hmm, what else, well, like Kris wrote, the sponner is not a great fit for all brake levers, so you can trim the lever, wrap the spooner in tape, or use a heat gun to distort the lever to fit. KH has a new universal spooner coming out, I’ll probably wait for those and make due with the others for now.
Avoid carbon brake levers as they are fragile, avoid ultralight brakes and custom machined brakes that would be expensive to fix if they broke. Simple is better when it comes to brakes.
Magura makes some decent disc brakes, Martas are often on sale and not too expensive, maybe $100 a set. Kris is using the Marta, so it must be worthy 
I’d add that it’s worth not dismissing braking power. After a decade of using Magura rim brakes and a year on disk brakes I’d somewhat disagree with Ben’s suggestion that HS33’s have lower power, as long as they are properly set up - being rim brakes they also scale in power with wheel size. But the predictability of disk brakes is certainly nice, particularly in the big wheel sizes.
Anyway re power - I find I really pull hard on brakes hard in some situations where you have lots of grip - full power needed. Plus any extra power will reduce fatigue when dragging the brake on long descents. To avoid disk rotor vulnerability for muni, if you need more power it’s probably worth moving to a stronger brake before you move to a larger rotor, as well.
Lastly on brakes - it can be the pads that lack power not the brake itself, and brake pads get stronger when they break in. Options also exist on some brake pad brands between greater performance versus greater durability.
Kris
Note - my comment on braking power notwithstanding, I do agree that we don’t max out braking power on unicycles compared to what’s required to stop a 40 lb DH mountain bike.
Also, I don’t think ultimate braking power is necessarily directly related to modulation (e.g. grabbiness)
Using a 180mm rotor works quite well for me (first MountainUni, now an internal Shimano disk). Both were used with a Shimano SLX brake.
So far i never hit the disk and the brake never disappointed me.
The only thing i noticed: the disks get really really hot sometimes! So i dont know if using a smaller disk + more braking power will be very good. I will stick with my 180mm 
This is something that’s bugging me about the new uni disk brake adapters. Given almost all brakes now come in a post mount version why are the uni disk brake mounts all IS? Why not make a post mount version, thus getting rid of the need for adapters? ISTM there is no advantage at all to using an IS mount with an adapter - it’s just added weight and complication.
I rode last night in the rain, Magura rim brakes, not a whole lot of stopping power even with two fingers, hopefully this is my last week of rim brakes.
I think the amount of brake you need depends a lot on how you use your brake, I find that brakes can put me off my line, esp on tech downhills, so I glide the brake and use pedal pressure for control; last night had me thinking that longer cranks may be in order 
At 200# riding the Oregon, I have never found the 160mm rotor to be underpowered, even when riding in very wet conditions; like in Oregon.
Bigger rotors and stronger calipers have one advantage in that you might be able to get away with one finger braking in certain situations where two finger braking would be required on a smaller rotor, but then that same big rotor is going to be an overkill in light braking situations.
Disc brakes are smooth and they brake when wet.
Yeah, that would have made sense, but the only reason I can think of is the ability to shim the brake in or out depending on use.
Seriously, on a uni? I can’t imagine you’d get a rotor that hot, but if you are then it might be time to try a metallic pad or change to a better rotor.
Well, this and the ability to run a larger rotor just by switching out the adapter. I’m not sure if that would be possible if the caliper mounts were not IS. Not many will want to run a larger rotor but I could see it being useful on a 36".
Yes it does get really hot - same as a bike. You can hear the hiss when you touch a wet glove to it =)
But I have never even come close to hitting or touching it when riding.
Kris
Given that most MTB forks are now post mount and many people use discs larger than 160mm, such adapters obviously already exist (and are still simpler than an IS to post adapter) http://www.chainreactioncycles.com/Models.aspx?ModelID=13717
Meanwhile I should have thought the majority of uni riders will be happy using a 160 disc, and will be using a standard hub hence no need to shim, and a post mount would make far more sense rather than having to add another adapter.
Actually that makes sense - most use of a brake on a uni is presumably dragging, which is how you really heat up your rotors - in terms of energy, braking from 30mph on a bike is only equivalent to dragging a brake down 9m of vertical.
Like Kris said
The longest ride with my new setup and without a pause was down 550 meters in attitude in around 2km. Braking all the time of course 
I only touched it with my gloves on (Specialized Fortress, very nice leather/carbon gloves) and i could feel the heat through it.
Im using the Koolstop brake pads for the Shimano brakes: http://www.koolstop.com/english/disc_pads.html (the red ones, semi metal)
I also never had problems with touching the disk. I never hit it on the external setup and dont expect it to get hit on my internal now.
@Nurse Ben
Thanks for this. I figured the front would be better for cable length, but it was the fitting information I wasn’t sure about.
This info has been very useful, I’m sure I’d have ended up with the wrong bits if I hadn’t had it. In the end, I’ve gone for the Tektro Auriga Comp you linked, plus the adapter. I see in other threads you’ve got yours set up now - how’s it worked out? Was it straight forward to fit, and are you getting the all-round braking performance you need? (I’m a little way off having my new uni assembled.)
I rode the new set up last night, the Aruiga is a fine brake, esp for the price, set up was easy, great power without being grabby. Mine actually came with adaptors, so maybe your brake will have one as well and you can send the extra back. I do notice a couple things, first is the disc brake set up adds about 250 gm to your uni; heavier hub, heavier brake. Personally, since I use my brake a lot, this is a good trade off. In contrast to Maguras, they need zero manitenance, I can brake with one finger vs two and my stopping power is greater, the glide is so much smoother, and they work when wet.
The wheel build is interesting, having been building symetrical muni wheels for four years, to build a “bike styled” dished wheel is wierd, esp because one side of the spokes is so much steeper and tensioned considerably higher. I do think that the dished wheel is strong enough for tpical muni/road use, but if I was doing huge drops where I could be landing sideways, I might stick with a standard “symetrical” wheel build.
The 29er built up much easier and nicer than the 26er, long spokes make for easier tensioning I suppose.
In terms of line length, a front brake line is long enough for most 24-29 muni, but if you use a long handlebar or you are riding a 36er, a rear brake with the longer line would be better.
Clearance is scary tight, 2mm between the caliper and the spokes, a bigger disc would improve clearance, but I did not have any problems so it’s probably fine. It’s possible that some brake might not work. The D’ Brake adaptor is awesome, no mess, no fuss.