I have a 1983 300SD that I think has a bad vacuum pump. The last time the car was driven, my wife said the brakes were really hard to apply and we had a heck of a time shutting it off manually. I unhooked the main vacuum line the goes straight to the vacuum pump and I couldn't pull a vacuum with my Mitty Vac. Is this a good test? I am not too keen about starting the engine, because I have heard that the bearings in the vacuum pump might get into the timing chain and make a real mess. Anybody know where I can get a new pump also?

Before you decide that the problem is the pump, check the check valve. It is located on the pump where the main vacuum line attaches. If you remove the check valve, you can often see where is has failed (broken parts inside.) From your vacuum test, sounds like the check valve. The valve should allow air flow into the pump, but not back out. Good news is that you can remove the valve with the pump still installed on the engine. If it turns out to be the pump, you can usually overhaul it. You don't want to buy a new one.

The proper way to test the pump is to connect your MityVac to the vacuum port at the pump (use stretchy natural rubber tubing if necessary, the kind sold for tourniquets). Start the engine and see if the gauge on the MityVac reads down 25 - 30 inches and is steady.
I just replaced the Vac pump on my '85 300TD before it disintegrated, it was getting noisy, a clear sign the bearings were shot. Some prefer a rebuilt, I preferred to go new because the last one I tried to rebuild had a little bearing in it that was bad and wasn't available as part of the rebuild kit (it is the diaphragm only) so I wasted a lot of time trying to get a part that wasn't available.
And you're smart in not running the car until its replaced with a good one (presuming it is really bad) because they can and do cause engine failures.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

I will test the check valve at the pump fitting instead of at the first joint, now that I have a metric 6mm hex bit, because I am curious about the check valve, but I think I would be farther ahead with a new vacuum pump also. I am getting a squeak, squeal noise that soundes like a water pump going out if you know what I mean. Could a vacuum pump going out sound similar? I didn't have a mitty vac to measure the pump a couple of weeks ago, but I pulled the line at the same place, first joint, and stuck my thumb over the end and I had good vacuum. Did the same thing yesterday when I was trying to get it to shut off and I had very little vacuum. I had the manual shut off switch adjusted properly, I had tried it a number of times, but yesterday it wouldn't work enough to shut it off. Had to put it in gear, have the wife push the lever, and I had to smother it to get it shut off. Whew!

My recommendation would be to remove the check valve from the pump. It is the nipple like fitting where the vacuum line from the brake booster attaches to the vacuum pump. They are prone to failing internally, in which case you will likely have loose parts come out in your hand. If it does not have any obvious damage, Try blowing through each end. Air should be able to flow in the direction of the pump, but not the other way.

Thanks for the help!. The outside housing check valve was OK. I removed the pump from the car, removed the outside cover and my problem was staring right at me. One of the three check valves had broken apart and the piece was in the housing. Still trying to find a price on a new pump to compare the rebuild/replace option. The rebuild kit is around 60$ and can be complicated I hear.

If the rocker arm is okay and the cylinder is not scored or other wise distressed, just put in the valve kit. It is not hard at all. The only real challenge is getting the rocker spring compressed when you reassemble it. Or you could leave the piston alone and just replace the valve and seals. Really a piece of cake. Don't forget to procure a new pump gasket. It is not usually part of the kit.