I'm no expert, but I don't think the engine RPM should change if you step on the brake pedal. The brake booster vacuum should be a closed system. I can't explain your fuel smell.

I ran into a weird situation with my 250SE 108 where this same situation happened and it turned out the seal between the booster and the master cylinder was leaking.
I didn't understand the connection between the brake application and the RPM increase until I started the car one day and was getting clouds of smoke out of the tailpipe, (I initally thought it blew a head gasket!)then there was no fluid left in the master cylinder reservoir. The booster seal failed and over time got worse until the engine vacuum sucked all of the brake fluid into the engine. I figured this out when I saw that the little plastic bottle in the booster vacuum line had fluid in it. I blocked off the booster line and restarted it-no more smoke. I replaced the seal and the master cylinder. The car idles and has run perfectly ever since.

Again I'm no expert....