Boost Gauge Reads Full, Handbrake Light On

[klebestift]

Not sure if this is the right section to be asking this.

Recenlty purchased an R34 GTT, and soon after the handbrake light came on while driving around (dimmer than it is when the handbrake is on).

I did a few searches and first stop was to check the brake fluid, but it was already over the max line (see first pic). I bought a little bottle of DOT4 and put a bit extra in anyway (second pic), but the light has stayed on.

What else could the light be indicating? Could it be lit because the brake fluid is over the max level? Previous owner only recently had the brakes replaced and they still feel quite good - stronger than any other car I've owned.

Second - possibly related - problem is the boost gauge. The sender under the bonnet is unplugged (both power and pressure - there is a screw sealing the tube from the manifold) but the boost gauge changes depending on the state of the handbrake, as follows:

Handbrake on - Handbrake dash light full - Boost gauge all the way down
Handbrake off - Handbrake dash light half - Boost gauge all the way up

From memory when I test drove the car, the boost gauge was all the way down all the time and the handbrake light was not lit.

I tried reconnecting the electrical connection on the sender under the bonnet but it made no difference, I've got some vacuum hose to try reconnecting the pressure line as well, but I don't think thats going to make a difference.

There is an aftermarket head unit and I read somewhere they can tap into the handbrake signal, so is it possible this was stuffed up and is causing all the issues? I'm pretty sure it wasn't happening when I test drove the car, so why would that suddenly change? Any ideas where I should start looking?