I have last generation Shimano 105 stuff on my road bike, all 5600 bits. I probably need a new back brake calliper but the current 5700 says it’s only compatible with current generation of levers.

A bit of Google action suggests the cable pull ratio is very different so that’s the problem.

Can’t find anyone with 5600s or older Ultegra 6600s left; anyone tried mixing 5600 and 5700 or would other brands work (probably SRAM as Campag QR is at the lever end and I need to be able to get the wheels in and out easily).

I’ve got 9 speed Ultegra calipers that are at least 10 years old on my winter bike, and am running new 105 hidden cable 10 speed levers on the same bike. No issues at all.

I’ve got the same 105 levers on my good bike and they work with 10 speed ultegra brakes.

these levers;

these calipers;

I’m using older 105 levers (5700 ? at least 5 years old anyhow) with 2013 Tiagra callipers. It works okay, not sure if it’s as good as it was with the original 105 brake but it does the job.

6600 brakes work fine with 2013 10 speed Tiagra.

So new levers/ old brakes is OK, but new brakes/ old levers not so good?

Bollocks!

Im in a similar position. Apparently 5700/6700 series brakes (presumably 6800 too) will work but they will feel a bit spongy compared to 5600 brakes with 5600 levers.

I have 5700 105 levers and have just fitted 6800 ultegra calipers. Very straightforward and works a treat.

Seems eBay is the place to go, NOS 5600 brake ordered for £21 – disaster averted.