Doesn’t matter, just be careful as some rear adapter can be used for a front brake but with a different disc diameter !
140 IS (international standard) rear = 160 IS (international standard) front
This ^
The only other thing is hose length and sometimes the levers are sided but us unicyclists care not for such things.
Another thing to consider is the angle of the reservoir some set ups while fine on bikes let air into the system due to the angle of the lever under the seat. Ideally you want a radial mounted master cylinder (runs from the lever hinge to the clamp rather than one that runs straight forward).
Hopes are losing their spot as the brake of choice in Trials now though, Shimano seem to have come out of nowhere with some extremely well priced and effective brakes. I know some people swear by Hopes but from my experience unless you buy brand new they tend to be a pain in the ass.
I’d definately agree with either a low model Shimano or the MT2 but also if anyone spots an Echo TR disc going cheap it’s worth picking one up, the discs are a bit hard so take some bedding in but other than that they are superb, also run mineral oil and you bleed them like a magura which makes life very easy. I run Echo TR calipers front and rear with some £15 Shimano levers on my trials bike and they are the best brakes I’ve used so far. So much so that when I get a muni frame sorted in the next few weeks I’m gonna source another setup like this to go on!
Mechanical disc brake
I see mechanical disk brakes on MTB’s also. Are these usable at a cheaper price or are they not worth the effort ? Seems like they would be easier to mount due to no hyd. hose or bleeding but maybe not.
They are big so don’t fit on the spirit crank disk setup. Not tried on the oracle hub but should work.
Hydraulic brakes are fit and forget as self adjusting where cable ones need to be adjusted for wear. I have been using discs on bikes for years the really are trouble free once setup.
Get a nice cheap Shimano setup ideally one with the servo wave lever as they are good value and disc rub free
Ben has tried to get a mech brake working on an Oracle hub setup and was unable to find one that wasn’t too thick. See the first 3-4 pages of this thread: Disk Brake choice for new Nimbus hub
I tried putting a Tektro Aquila on a MountainUni crank based setup but it was about 2mm too wide (cleared the crank but not the spokes). I’d be very happy to learn of a significantly thinner mechanical caliper.
Noe being an MTB rider for many years so was about when disc bakes genuinely came about I remember these puppies
Hydraulic Cable actuated brakes
Time to start searching retro bike parts 
Hayes?
I have a line on a Hayes El Camino or a Hayes “9” (?), both front brakes. Has anyone got experience with these? The price is good (~$65) compared to what new brakes cost so I thought I’d try one - unless they’re known not to fit or are total crap. Reviews on pinkbike are only moderate but they may reflect expectation based on new prices which for the ElCamino were really high.
If they’ll work I’ll be installing on a KH 36 (with mount), with a 180mm rotor.
How much are Shimano Deore M596 discs on your side of the pond? Dirt cheap over here and they work great! Would definately recommend one.
I recently installed a Shimano XT M785 brake with an SLX 180mm rotor on my KH26 and it works fantastic. This is my first experience running Shimano brakes and they are proving to be rub free and have great modulation. I’m used to the Hope Tech V2’s with floating rotors(203mm/185mm)on my MTB and I’m happy to say that I’m sold on these newer Shimano brakes.
P.S: It was really Feisty’s “Servo Wave” sales pitch that turned me on to Shimano brakes. Thanks
Just a follow up on the Magura MT2:
They do not stand up to long term use, both sets of my MT2 have lost the ability to adjust the lever position. It is not something that can be fixed, the design is defective. DO NOT BUY these brakes.
Also, my spare brake, a Bengal Helix which came on my Oregon is no longer operational, the lever won’t return. It has minimal use so I’m assuming it just froze up while in storage. So it’s been canned, yet another brake that did not stand up to the test of time.
On a positive note, I have been using a Shimano BR M596 which uses some of the newer XTR tech, two finger lever, great power, and only $80 USD (no rotor or adaptor).
D Brake and Oracle frame compatibility with the External Rotor standard:
Unlike the hub mounted rotors where the caliper and D Brake are mounted on the left side of the uni, for a crank mounted rotor you mount the D Brake on the right side of the uni. Depending on the thickness of the crank spacer used, you may need to add thin washers between the IS adaptor and the D Brake in order to get the caliper to line up with the rotor. Otherwise it works quite well.
Does yours also lock in one position sometimes? Sometimes after a fall my lever will get locked up and it makes a horrible sound trying to unseize it.
Ive noticed that the adjustment is pure shit after a while, it works but has A LOT of play in it.
Lastly have you tried replacing the lever blade? You need special tools from magura just to do it… so stupid.
The MT2 levers are garbage, the lever adjustment is the worst design ever, the clamping mechanism is shite, but the caliper is pretty good, soooo I think I’ll find some levers from a different braking system and do a cut and past.
lolololololololololololololololololololololololololololololololol
Never a dull moment with you 
We got a winner!!
I called and talked to the tech folks for TRP, he measured the caliper and it sounds like a good fit:
Caliper Thickness 40mm
Caliper inside and outside dimension from rotor center ~20mm
For comparison, a SHimano M596 is ~42mm thick and measure 25mm to the spoke side and 17mm to the crank side. On my Oracle 29 with Spirits I have >20mm spacing on either side of the rotor.
They are located in Ogden, which is on my way to Washington State, so I’ll be picking up a pair next week 
BTW for the above ^^ that’s a mechanical dual actuating disc brake designed for cyclocross!! Available with 140 or 160 rotor, but clearly you can use any rotor you want with the correct adaptor. Dual actuating means that both pads move, which is generally a high end disc brake thing and not common for mechanical brakes; BB5 and BB7 are single side actuating.
I was very tempted to have them overnight a set to me, but being as we’re leaving Friday morning, well, I just figured it could wait a couple days 
It’s pretty exciting to have mechanical disc brake options, esp for those folks hard on brakes as this is a way more durable system from a field repair standpoint, and cable length and pad clearance adjustment will be easier.
Here’s another cool brake from TRP:
This one is also cable actuated, but at the caliper it is hydraulic, the resevoir is located on the caliper, not sure on the dimensions, but it’s a neat twist.
The bengal is dead
After a very short service of under a year my Bengal Helix 5 has died. I’ll spare you the details but, succinctly put, the Bengal has developed a myriad of problems over its lifetime and has finally came to its time to go. Nick’s (of UDCUK) Bengal also died recently which makes me think that these brakes are a peice of crap.
DON’T BUY A BENGAL
It will leave you poor and depressed
(or a the very least a bit pissed off) 
A lot lighter than a BB7, paired with a light lever it will get close to the weight of a high end hydraulic. Interesting.
There is one issue with the lever though - as a road/CX brake it’s designed for use with a drop handlebar road lever. Those pull a lot less cable than a typical MTB V-brake lever, so if you pair that caliper with a V-brake lever you’ll get a low mechanical advantage on the brake - ie a lot of lever force needed for a given braking force. You’ll need to pair it with an old-fashioned canti lever (canti levers have a lower cable pull).
Good news. I saw a few press releases on these back when they were announced, good to see them become available. When/if my inexpensive hydros go I’ll probably look for something like this.
I got a Avid elixir 3 for free 