Route preference for one address

[kc atgb]

Hello,

I started using bird some days ago from zero knowledge. With the different docs
I found all around I understand a little bit more how that works. 

My configuration is made of 2 sites connected by 2 seperated links. Each link
have a dedicated vpn connection for internal routing. One nominal link and one
backup. 
In case of failure, the switch from one link to the other is working. 

What I want to do now, is to force the routing for one specific address (/32)
via the backup link whereas the rest of the /24 is using the first one. And
in case of failure of the backup link, the /32 should use the first one. 

I tried different things and from what I have found, my configuration is
working partially. I mean there is no fallback to the first link if the second
is in failed state. 
I tried with static routing and export filter. 

With static:
protocol static s_special {
      import all;

      route 192.168.1.78/32 via 10.194.11.2 {
              preference = 250;
      };
}

The route stays active when the link is down so the traffic can not go back to
source. 

With filter:
filter export2kernel {

        if net = 192.168.1.78/32 then ifname = "tun1";

        accept;
}

The same thing happens. 

I took a look also to preference/cost/... but I am unable to achieve what I am
looking to do. 

So my question, what should I look for to have this working like explained ? 
What information should I provide if it is not clear ? 

Thanks in advance,
Kay