Update: Brake light issue fixed. Turns out it was the brake light switch. The plunger that operates the switch needed adjusting as it wasn't making contact with the pedal, causing the lights to stay on. Fairly common oversight when removing a brake booster :facepalm:

Thanks Ian I'll give the booster and master cylinder area a good looking over later to double check for leaks. II'll write in again if there's any updates

[Edited on 12-04-2019 by VoicelessGrapefruit]

Rev fluctuation is consistent with a vacuum leak. ie. engine intake is open to atmosphere instead of being sealed.

You may be able to find it by spraying carb cleaner in the hope that some gets sucked in as it'll rev up if you manage to draw any in.

But yes, hiss when the pipes move isn't good news.

I spent the whole day doing the wet footwell fix on my Corsa C, exactly how the guy does it in this video including moving the master cylinder out of the way without draining the fluid and removing the brake booster:

https://www.youtube.com/watch?v=ChzIbEmGfgU

I got everything back together and thought it was a job well done, until I turned the key... The brake lights are stuck on when the ignition is on, and there is a hiss coming from the master cylinder/brake booster area any time the brake pedal is pressed and depressed. The engine's revs seem to drop a bit when the pedal is pressed also. I won't dare take it on a test drive as it's a safe guess I'll end up through my neighbour's living room wall if I do.

Everything was reconnected and tightened up properly and there appears to be no vacuum or fluid leaks, so I really don't know what the culprit could be. When I pulled the master cylinder away from the brake booster at the start of the job, there was a loud hiss which I thought was just pressure in the booster being released. Looking back I now think that was the first sign I ballsed something up.

In real need of guidance here as the car's basically a vegetable until this gets sorted. Have I simply just got air in the system somehow? Has the brake booster being damaged by the removal?

Cheers!

[Edited on 11-04-2019 by VoicelessGrapefruit]

[Edited on 11-04-2019 by VoicelessGrapefruit]