Badger6

I've got this 2015 Diverge (carbon frame) that is on its 3rd life...long story, not relevant here...I love the ride. But, as I break stuff, I keep replacing and upgrading, so here I am...

Frame has post mounts (2015...of course it does)
Brifters are the GRX Di2 as I state in the title
Calipers (current) are functional, original, and now leaky (O-rings are shot again) RS785

I'm done rebuilding the calipers....unless I am not....so I am here to get info and knowledge, not opinions, on mounting Shimano MTB series post mount calipers (XT if they work). Additional thoughts: I weigh in at 81kg (179lbs), 4 pot on front or 2? Stick with Shimano or look at a compatible brand (even if not Shimano "preferred), like Hope? Make me smart.

Elvo

Yes they are compatible and yes you can go Hope but SLX/XT is good enough. Some of them use the banjo bolt which is not used on the road calipers.

Cyclist0108

I have an XT caliper on one of my Ultegra "brifters". It is identical to my post-mount Ultegra caliper in every respect except the label and the bolt that holds the pads in place (Hex vs slotted head).

Hope gives the option of the 4-piston, but you lose the cooling fin pads. I think those are more important.

If you want new old stock Di2 ultegra brifters with post-mount calipers, you can get those for $199 from Universal cycles now. Or just canibalize the calipers and keep the levers for a backup or a new project.

Badger6

I'll never need the brifters. Next road bike (with discs) will be equipped "out of the box." I've GRX brifters on the gravel bike already, crash replacements for the older 785s I destroyed a month ago in a crash. The problem is the 785 calipers used to be good enough, but they just aren't anymore.

Poster above says the XT MTB calipers will work, good enough for me. Current set up is just not working so great. I ended up this morning getting some 4-pot (piston) XT (M8120) for the front, with finned pads (agree, these are critical)...I ride plenty of gravel here around Brussels, but take several trips a year to the Black Forest and Alps with some long and squirrelly gravel descents. I did enough reading the last few days to be convinced that this is the route I should go. Standard 2-pots out back, just made sense. I'll update once I've got them installed with any important notes.

Oh, and anyone thinking of doing this, the newer calipers use different banjo bolts than are supplied in the brake hose kit, and there are none provided if you buy just the caliper. So make sure you source the correct bolt, because the M8100 and M8120 use different ones.

gus6464

All you need is a caliper that uses BH90 hoses, that's the only requirement. Basically anything MTB except for the older Deore and Metreon won't work as those are BH60. As for road you can use any caliper except the very first generation of road hydro as those were still based on BH60 hose.

When I say older Deore I mean truly old Deore.

DorkDisk

The two main hoses are BH59 and BH90; they are compatible with each other, one just had to use the proper olive for each end. BH59 is a "detuned" version that has a lesser on/off feel, and more of a squishy feel vs BH90. MTB and road calipers are fully compatible, first Ultegra units were rebadged XT.

Badger6

Apologies for the long post...

Completed the upgrade, because thats exactly what it ended up being by the time it was complete after 3 hours of disassembly and reassembly of the brake system. For reference, the Shimano brake system build I was seeking was "limited" by the post mounts for the calipers on the frame, so my options were severely limited unless I got a new frame. I did look for the Hope RX4, but the post mount version was unavailable everywhere I looked, and it seems to be discontinued...and by staying with Shimano calipers I had pad options that were finned to assist in heat dissipation on longer descents.

I mounted the current generation XT brakes: BR-M8120 on front and BR-M8100 on back with metallic compound finned pads to ensure I've got reliable braking power and pad durability as we are about to enter winter which is very rainy here in Flanders. The rotors are the current Ultegra series RT-800s. Hosing is BH90 with Banjo fitting at the caliper.

Getting the brakes functional took a bit of work, but nothing unnatural or unexpected.
1) Only real mounting issue I ran into was with the hosing, which is quite stiff (a good thing to prevent expansion and provide a more predictable feel), needing to make some tight "S-turns" from the banjo to the holes in the frame. I just warmed it up with a hair dryer to get it a little more pliable, and carefully routed it so that it was not kinked. It also made mounting the calipers a little bit tricky as they didn't want to line up to the posts, but again, being careful and taking my time, I was able to get the calipers in place and aligned so the rotors would turn freely.
2) Bleed was simple, and I took more time on this than I normally would because of things I had read about brake response when mating MTB calipers to road style levers. I just wanted to ensure I had removed any variable that might cause issues later.
3) Fully bled, the lever travel was still excessive for my taste, traveling more than 1/2 way before engagement, and even on the stand, it seemed that the brakes were not very powerful, so...
4) I dropped the wheels, one at a time, and began pre-loading the pistons, giving 3 rapid succession pulls on the lever, then putting the wheel back in and testing the travel and bite. On the front, I did it once, and the lever became full firm with travel in the range I desired (about 1/3 distance to the bar before the pads engage, and 2/3s to stop). On the back, I had to go through this cycle twice. Now, why did I not just adjust the free stroke screw? Because in my experience this works great when the pads are brand new, but as they wear, the screw's ability to take up the free stroke diminishes, but using this "trick" retains that option as they wear....it works for me. It's not as precise, but it gives me a lever feel I like.
5) Initial test of the brakes confirmed they worked as expected, but clearly required bedding the pads. I ran each end through 5 heat cycles, and the bite was tremendously confidence inspiring. The power of the brakes went up dramatically after the first 2, and then continued to improve after, but not remarkably so.

To build this setup, there was no "easy joint" option, since I was mating MTB calipers purchased as separate components to the existing GRX levers. Shimano doesn't explicitly say this won't work, offering no indication that MTB calipers and road/gravel levers can be operated together. But, Shimano also bills these levers along with all current series road levers as being equipped with their servo-wave, first run in MTB levers, so it reasoned the calipers would be just fine. I bought the calipers and hoses separately, and looking at the scope of delivery, it did not indicate that either of those would include the required banjo bolts or o-rings....so I bought those separately (and they are different for the two caliper models, so if you do a similar setup, ensure you get the right bolts). To my surprise, the calipers arrived with the bolts, but no o-rings. When the hoses arrived, they came with the o-rings, and the required olives and barbs. So now I've got some spares.

I have not had a chance yet to put the system to work in the Zoniëwoud on the gravel and single tracks, but I have no doubts it will live up to expectations. The 4-pots (8120) might seem excessive, especially because Shimano specs them as trail/downhill, with the 2-pots (8100) spec'd for XC...but, using this same F/R caliper setup on my XC rig, I find they are powerful and offer a lot of modulation and better feel. Some reviews indicate an on/off feel, but my personal experience does not track that. I weigh 80kg (about 180 lbs), and I tend to get carried away with myself at times, so having a more powerful braking setup with a greater ability to modulate, especially on the front, is exactly what my wife needs so I don't kill myself in the forest.