Flat mount calipers for rs685 levers

I have an old pair of rs685 ultegra hydro shifters currently with post mount calipers. I’m looking to put them in a bike respiring flat mount calipers. Hoses are bh59.

can I use any flat mount shimano caliper (ie I have a spare grx in a box somewhere), or will the newer models that run bh90 hoses be incompatible

my guess is as long as I have the correct olives and barbs all will be fine, and the only difference will be slight change in lever feel. Can anyone confirm.

thanks

I did this a couple of weeks ago when I did a frame swap and went from post to flat mount. I have RS685 levers with Ultegra 8170 flat mount callipers and BH59 hoses. It works perfectly, no complaints.

Perfect thanks

Shimano brake compatibility chart

Looks like RS685 levers are compatible with flat mount RS805 and RS505 calipers using BH59 hoses and with flat mount R9170, R8070 and R7070 calipers with BH90 hoses.

Hope rx4s go nicely with 685s if you’re tempted by another option. Last time i checked they actually weren’t that much more than buying shimano flat mount calipers once you accounted for the fact that they come with two pairs of pads.

Yep, works fine, I’ve got RS685s and I upgraded my old post mount ultegra calipers to new 8170s. I did need to change the hose from BH59 to BH90 though.