I tried swapping my stock STI double diaphragm brake booster with a single diaphragm booster (26402FA071), to try and stiffen the brake pedal a little. I retained the stock 1"-1/16 master cylinder.

However, after the install, the exact opposite happened. My brake pedal travel became long and soft and super sensitive. Just a light tap and the car will nose dive and stop.

Could the installation have been messed up in any way? It doesn't seem like air in the brake lines because the brakes bite really hard and early, but the brakes feel overboosted now (when it should be underboosted).

And I actually tried with two different units of the single diaphragm brake booster, one was reconditioned and the other was brand new. Both had the same soft and oversensitive feel (although the brand new felt slightly better).

Any ideas what could be going on?

I should also mention that my car is a RHD but I could only get hold of LHD brake boosters with single diaphragm, so had to use a small extension hose to extend the vacuum hose feeding the booster, as well as extend the plunger rod where the brake pedal connects too. However, as far as I can tell, the pedal ratio should not have been affected since the length of the plunger rod has been modified to be the same as the original RHD booster.