jonathanf2

I'm thinking of swapping my GRX 400/105 setup over to my post mount CF frameset. The fork is flat mount (will keep front GRX flat caliper), but the frame is post mount. Which hydraulic post calipers would be compatible with GRX shifters? I was thinking either the Shimano BR-RS785 or XT M8000 calipers might be viable options? The other idea was to order a flat-to-post frame adapter from A.S. Solutions and keep my GRX flat mount caliper. Though aligning GRX hydraulic brakes is a pain and adding another alignment variable might make it harder to adjust.

Any suggestions would be appreciated!

DorkDisk

RS785 have been very good to me with GRX800.

One thing to note about M8k is that the hose enters from the outside, whereas the road calipers have the hose entering from the inside.

jonathanf2

Thanks for the information! Do you know if could I use the same existing GRX hydraulic hose setup with the RS785 brake caliper or is there any changes with the hardware that you're aware of?

Chilepines

Strangely enough it appears that not all RS785 brakes are the same. I am actually selling a set on eBay - those use a banjo bolt attachment. The calipers in the pic above seem to have an inline connector. It's not clear from your post which brakes you currently have and that will have a lot to do with your ability to make a direct switch. There is also the issue of BH-59 vs BH90 hose - the GRX levers use BH-90 and the RS785 calipers (which are older) use BH-59.

DorkDisk

BH90 is a MTB hose and the new road hose. BH59 was the first iteration of road hydraulics and used a thinner walled (same outer diameter) hose that "flexed" more to give a more "road" feeling. The olive depends on the hose one uses, as the diameter is different to fit the different inner diameter of the hose.

BR735 was well known to be a rebadged XT, Shimano has yet to make the two systems incompatible, MTB bikes are now using flat mount also.

I use BH90

jonathanf2

I just checked, my GRX 400 brake calipers are using SM-BH90 hoses. I also have one of those connector valve that allows me to disconnect the hose without removing the bar tape.

So could I just swap the hose that connects to calipers to that connector or would I have to route a completely new hose line to my brake/shifters?

Polaris OBark

I had the opposite problem, so now I have two orphan Ultegra post mount calipers ( BR-RS785 ). They are essentially clones of the XT 2-piston calipers. In fact, on another bike, I had to replace one with an XT caliper. They are interchangeable. (I am using the old 2014 Dura/Ultegra Di2 brake levers with the most recent generation flat-mount Ultegra calipers.)

As far as I can tell, all the hoses are standard, so you shouldn't have to change anything.

jonathanf2

I just realized when I did my last internal cable hose routing, I actually used the BH59 barb and olive despite having BH90 hoses. So apparently fit isn't an issue and I haven't had any issues braking, nor any leaks. I'm guessing switching to a RS785 post caliper should be fairly straight forward.

On a side note, I'm still debating if I should try the flat to post adapter solution. If I go with the RS785 caliper, I'd have to buy 2 different brake pads if I run the GRX fork mount caliper. Though the plus side is the RS785 pads seem more easily accessible.

jp911

There are flat-mount caliper to post-mount adapters available. Wolftooth just released one and there's a Canadian company that makes them as well (name escapes me).

jonathanf2

I had no clue Wolftooth recently announced this. I'll definitely give this a try with my GRX caliper and it should ship a bit faster. Thanks!

Edit: Never mind, with the Wolftooth since my post mount is 160mm native, I'd have to run 180mm rotors. So I'm back to my original choices!

jonathanf2

Just an update, I ordered the 160mm A.S. Solutions FM to PM adapter. After some thought I didn't want to deal with 2 different brake calipers and brake pads. I'll report back once the adapter arrives and I'm able to install it. Thanks!

Polaris OBark

I would have sent you mine.

Sorry, I should have made that clearer.