Shimano GRX Hydraulic Disc Sub Brake Levers with 105 groupset

Question

The GRX groupsets seem to be compatible with common road groupsets (Tiagra, 105, Ultegra) which was also pointed out in this answer. Furthermore, a brief news article by Shimano states that the brake and drivetrain components between GRX and road groupsets are interchangeable/interoperable:

Fully Compatible Parts

As long as you maintain your current drivetrain speeds (10-speed versus 11-speed), the following GRX components are fully compatible and interchangeable with your existing parts: Shift levers, chains, brake calipers, bottom brackets, cables and housing, and Di2 batteries and accessories.

However, some additional brake levers BL-RX812 for the upright position were introduced with the GRX series about which I cannot find any explicit compatibility information.

As Shimano's STIs combine brake and shift levers, I am not sure whether

Shift levers

refers to STIs (including the brake levers which I am concerned about).

Are the brake hoses of 105 STIs compatible with the GRX ones? Could I extend the road groupset brakes (105 or Ultegra) with these sub brake levers resulting in a setup consisting of 105 drivetrain, 105 STIs and GRX sub brake levers?

Answer 1

Shimano has a great compatibility website.

This is what I have found. They seem to be compatible with pretty much any hydraulic STI from Shimano and the 105 STIs (ST-R7020) are listed as compatible.

Answer 2

In looking thru Shimano's latest compatibility chart (specifically the section of "Compatibility of Shimano hydraulic disc brake hose") I too do not see specific reference to the BL-RX812 and the BH90 hose. However, looking thru the RX812's Dealer Manual, it clearly states that the BH90 hose is to be used with the levers and calipers dealt with in the dealer manual. Specifically, SM-BH90-JK-SSR (see pg.7 of the dealer's manual). It also sounds like the BL-RX812 sub brake lever will come with the appropriate hose to continue on from the sub lever to the STI.