Solution to GRX Lever Compatibility With Post/IS Brakes

by Jeff Whitfield on | 10 Comments

Tips Gravel

Shimano GRX lever and hood
Shimano GRX lever and hood

In a previous post, I talked about the upgrades to my Marin Nicasio which included a full drivetrain upgrade to Shimano GRX cranks and derailleurs. One of the problems I ran into had to do with the GRX levers.

I recently talked about problems surrounding flat mount brakes and getting them to work on a frame that only accepts IS or post mounts. In a nutshell, if your bike doesn’t accept flat mounts and/or can’t be adapted then good luck getting any of the new Shimano GRX brake calipers to work.

Hope is not lost though.

In that same post I made about flat mount brakes, one commenter mentioned possibly using a different brake caliper. I can’t believe I didn’t think about this myself. The idea would be to use a MTB caliper like the Shimano SLX 2-piston brake caliper. The goal is to use a caliper that can be post mounted which in turn can be adapted to IS mounts as well.

Technically, this should be possible. However, you’ll have to make for damn sure that the GRX levers support the pistons. If they don’t then you might be setting yourself up for some serious brake failure. If you’re really dying to upgrade to hydraulic brakes and can only use post/IS mounts then this might be a possible solution. However, do so with caution!

I’ve already upgraded my bike using Shimano 105 levers and TRP Spyre brakes. They’re not as good as hydraulics but, after a bit of tweaking, they have plenty of stopping power. I likely won’t bother with hydraulics this time around. Instead, I’ll just wait and upgrade my frame to one that accepts both flat mounts and thru axles. That combination in my opinion is a better way to go.

Post a comment!


I fitted XT callipers to 105 hydraulic brakes (i read the brake fluid movement was similar), and that worked a treat. Great powerful brakes for a commuter, and the pads last forever. Problem i have is the RHS shifter is smashed so now looking for a replacement... which is proving tricky for a 10 speed. Hence landing on this page re GRX-400. Everything i've read so far looks like it will be the same as my 105s.


Interesting. That's some damn good info! Good to know that 105 brakes work with GRX shifters. Have to remember that going forward. That said, my next bike frame will probably use flat mount brakes anyways so this will probably be a moot point. Still good to know though. :D


Yep! That's exactly what I learned too! I'll likely go that direction at some point. However, one thing I might look into is using an XTR caliper instead. I think the braking power on those is likely to be better.


Hi Jeff, I'm at the similar situation. I'm planing to upgrade my Nicasio with ST-R8020 lever. I found the BR-RS785 can work with BH90 hose and GRX/8020 lever in shimano's offical compatibility. (

The BR-RS785 may be a solution.


Hi Jeff,

I have another solution, which works like a charm on my bike. They're brackets to convert flat mount calipers to fit a post mount frame. It doesn't work on every frame, make sure to measure.




Ran into this same issue with a 2018/19 All-City Cosmic Stallion frame that I had built up as GRX. Local shop installed Shimano Ultegra RS-785 brake calipers and they work like a charm with GRX brifters.

Good luck!


That’s good to know! But, I’m curious...what would the difference be between an Ultegra and an XTR caliper in terms of stopping power?