New pads, old discs - pedal travel bad when engine running

Question

Changed my pads/sensor on my Mini R56 this evening.

Reused old rotors (discs) since I only changed those around 6 months ago (I didn't change the pads at the time as I bought the wrong ones hence the job being done now just before an MOT).

I did not change the fluid (did that 6 months ago).

I had a nice solid pedal before but now I don't.

I can pump the pedal with the engine off and it goes hard.

When the engine is running the pedal softens up a lot and there's significant travel now before the car starts to slow down.

I find that I then have to pump the pedal to build up pressure, but that pressure disappears quickly and I have to repeat this.

My procedure was sound: I ensured that the brake reservoir cap was off when I pushed the calipers back in. I stopped once the caliper piston was flush with its rubber surround.

All reassembled and pumped the brake before setting off on first trip.

Is this 'bedding in' and do I need to be patient or is something wrong?

Note I have checked the reservoir and the level is not going down.

Answer 1

Assuming there is no air in the hydraulic lines and you have the correct pads, which are also fitted correctly. Then you're probably just feeling the pads bedding-in. Basically when you brake, the flat surfaces of your new pads are now only riding on the ridges of your used disks, which means you need to press the brake pedal harder to try and get the same braking force, hence the softer feel & longer travel. It shouldn't take too long but your pads will need to wear-in to the shape off your older disks. Should the issue worsen or persists more than say 30 miles or so then further investigation may be required.