80 300SD valve problem

[benz300SD]

I'm kinda new to these cars and the engines, I have a 73 Mercedes 1113 truck and I've never had any problems with it so I got this 80 300SD. It was running great until I took it to do some shopping and on the freeway it all of a sudden started blowing black smoke and I lost 1 cylinder. I did some testing and I only have 150psi in #5 and 45% leak out the exhaust valve. I haven't torn the head off yet, but I was wondering if this is a common problem or if there is something I should be looking for specifically. I plan on getting the head off tomorrow to see if the valve is bent or burnt or what. And any advice on head removal would be appreciated.