Brake Pedal and Engine Noise

Hey all. I've got a 95/4.0 and today my brake pedal just goes straight to the floor. I've done a lot of searching and I just can't find the right answer. But there is a full sotry.

I picked up my jeep from a friend yesterday and I noticed the brakes didn't stop me well at all, and my oil pressure gauge was boucing all over the place. So I limped it home and went to work on it today.

My oil hadn't been changed in a while which was stupid on my part not to do. So first thing up I changed the oil and started it up. I noticed a clicking/knocking noise coming from a spark plug so I turned off the engine and tightened that and another loose plug (why would they be loose?) and my oil pressure has stabilzed. The noise is still there, but I cant confirm if it was there before or not. But my brake pedal just goes to the floor and hisses. After I turn the engine off, the pedal rises back up.

My question is what would have caused my brakes to go out like that? And what is the strange engine noise?

I'd also like your opinions on the best way to start troubleshooting these problems.

Thanks

Gavin