My long-term review of Shimano GRX Di2 will be hitting the airwaves here at Bikerumor very soon. In it, I mention the notable braking power of the sub-levers that live on the bar tops. These levers now sit in line with the main brake levers and the hydraulic fluid from the master cylinder flows through the sub-levers.
With the T-junction outlined in this patent, each lever can operate independently of the other. And, notably, only one of the brake levers requires a fluid reservoir.
Separating GRX’s integrated brakes
![Shimano sub-lever assembly](https://bikerumor.com/wp-content/uploads/2021/04/Screen-Shot-2021-04-12-at-12.11.51-PM.png)
Shimano devised an excellent in-line solution for adding a braking position to your handlebars for its GRX groupset. The sub-levers live on the bar flats and give you a great braking feel without having to plant your hands on the hoods.
Now Shimano is refining the braking dynamics of the system by adding an apparent inline junction. This T-shaped junction connects the hose from the sub-lever and the main brake lever, and connects both via a single hose to the caliper.
The question is, why add this extra component to a system that already works wonderfully?
But why?
As Shimano states in the patent, the patent outlines a system in which “an actuating device and an additional actuating device can be selectively used to operate a single hydraulic operating device.”
The junction features a pivoting guide inside that essentially closes off one of the brake master cylinder systems when the other is activated. For example, if you’re using the sub-lever to actuate the brake, the pivoting guide will swing to close the hydraulic fluid in the main brake lever’s system before it meets the junction. And vice versa: actuating the main lever will shut down the fluid system from the sub-lever.
Notably, the pivoting guide prevents whichever lever that’s not being activated from losing hydraulic pressure.
![GRX sub lever with junction](https://bikerumor.com/wp-content/uploads/2021/04/Screen-Shot-2021-04-12-at-12.12.22-PM.png)
You can also use an open hydraulic system lever with another open hydraulic system lever, or you can use an open hydraulic system lever (with a main reservoir) and a lever with a closed hydraulic system. An open system features a reservoir of fluid and air that accommodates heat expansion. A closed system lever features no reservoir, just a piston that pushes on a fixed volume of fluid.
T-junction benefits
That means you can use a different lever — rather than the GRX sub-lever — within the system. You could, for example, use a Shimano Deore brake lever as your sub-lever, which offers a larger brake lever and some more adjustment options.
And you should be able to configure the actuation ratios of each lever independently. You can make the sub-levers more positive and sensitive, for example, than the main levers.
A guess, based on my GRX experience
![Closed hydraulic system](https://bikerumor.com/wp-content/uploads/2021/04/Screen-Shot-2021-04-12-at-12.12.38-PM.png)
The second reason is a conjecture, and it addresses something I experienced in my year-long review of GRX. The problem: if you decide you don’t want or need the sub-levers after you’ve installed them, you’re going to have a bit of surgery to do in order to remove them. From there, you’ll need to re-run a hose just for the brake levers at the drops.
While the sub-levers feel great and do exactly what they promise, I found I didn’t really need them. Yet they’re still on my handlebars because I don’t want to do the surgery.
The solution: The T-junction. Of course, this assumes it’s possible to simply block off the input port that would otherwise accept the sub-lever’s hose. You can basically install this T-junction into the hose run between your main brake lever and the caliper. Then, if you decide to use the sub-levers, you can simply run a hose between the sub-lever and the T-junction in the main hose run.
At least that’s what I’m hoping you can do here. It certainly would be nifty to be able to add the sub-levers for a race or long ride when I want them, and remove them the rest of the time.
But primarily, it seems the GRX T-junction is designed to allow you to tailor the braking feel between the two brake systems and open up more options for various lever configurations (like using a closed or open system lever).
Patent research assistance provided by Wheelbased.com. Check them out for deeper dives on some of these patents and more.