Do cyclocross/interrupter/touring brake levers reduce brake performance? I have bullhorn bars, with a Tektro cyclocross lever and a TT style bar end lever as well, operating rim brakes. The calipers are Tektro dual pivot. The pads are cartridge style pads (I don't have the exact make and model with me right now).

This is the closest photo to my setup I could find. I have bullhorn bars rather than drop bars, but it shows the way the cyclo levers interact with the bar end levers.



I've been having trouble with braking performance, particularly in the wet. The levers don't bottom out (as I'm careful to keep them adjusted), but even squeezing very hard the bike doesn't pull up very quickly. I should also mention that I've tried removing each lever and haven't noticed any difference (although it can be hard to compare different days and different conditions).

I've had 3 different bike shops tell me that the problem is having both brake levers. They claim that it introduces too much flex into the system and braking power is reduced. I was very skeptical until all 3 people told me the same story independently.

This doesn't make sense to me. The cable is not interrupted and using either lever doesn't seem to compromise the brake line in any way. When you use the bar end lever, the interrupter is closed and I can't see any flex in the lever. When using the interrupter lever I don't see how the bar end lever can be causing any flex either.

Can anyone explain how having both levers would compromise performance?
The only way that interrupter levers could make much of a difference is if the installation requires extra housing (which would compress under braking, increasing the lever travel for a given braking pressure). If you're not bottoming out the levers, though, that's not the source of your problem (and from the picture I don't see why the interrupters would require additional housing anyway).

I think Daniel R Hicks is correct here--your lever/caliper setup likely isn't giving you enough mechanical advantage. You're squeezing the levers as hard as you can, but you're just not getting enough pad/rim force for decent stopping power. You notice this mostly in wet weather because there's less pad/rim friction when they're wet.

Finger positioning on the levers is one thing that can make a difference--keep them as far away from the pivots as you can. On my drop-bar commuter bike, for example, I need to ride in the drops during wet weather because I just don't get enough mechanical advantage with my hands on the hoods.

You might also see better braking performance if you switch to softer brake pads, or rain-specific pads like Kool-Stop Salmon or similar, though I've read that the Salmons tend to chew up rims pretty quickly when it's dry.