Here's another possible failure mode:

On 2 pumps in a 617 engine, I've had the piston shatter. They are cast aluminum and have often wondered if anyone would be interested in a run of machined pistons from 6061 alloy.

None of the regular part places (internet included) sell the pistons.The kit to rebuid the valve end is $45 on up to aroud $70 depending on who you buy it from. The few companies that sell the kit that replaces the arm and other drive parts; one wanted $265.00 for that kit. If you were to add the cost of a new piston it would be better to buy a new vacuum pump with a warranty.
I think the engine driven vacuum pump nees to be gotton rid of and replaced with some sort of electric on or at least a mechanical one that the parts of which will not fall into the timing chain when it fails.

On one of the "later" engines I went back to the "diaphragm" type pumps found in the early W123. The top of the air cleaner had to be swapped out for one with a vacuum fitting. The rebuild kits for those pumps are considerably cheaper also. They can, of course, lose their ball bearings too;)

One cause of a vacuum pump going south with catastrophic results is "for and aft (laterel?) play in the timing device due to a worn bushing on the intermediate shaft. If the timing device can be pushed back and forth more that 1mm, beware;)

After reading this thread I went looking and found as you say the diaphragm pump is cheaper to rebuild but I could find no one that sold the rebuild parts for the arm and bearing.
I also found there are 12 volt electric vacuum pumps made to power the Power Brake Boosters on gassers that have special camshafts or Blowers/turbos on the engines. But, new one are in the $270-$300 range.

Humm, I wonder if one couldn't take the guts out of a VP (to cover the hole) and hook up and 12v one. At least with the diaphragm type there is no piston to come apart just the ball bearings.

Timer End Play
I went back and took a closer look at both the CD and book Service Manuals and found it listed as: End plan of intermediate sprocket shaft: 0.05-0.12mm.
In section 07.1-240 of the Service Manual.
I assume this measurement is take with the Timer installed on the Shaft and with no Timing Chain on it to interfere with the measurement.
Repeat broke new vacuum pump

Good show, I dont think the timing chain would interfere the measurement, the intermediate shaft (and TD)will move with the chain on (at least several MMs). wouldn't it be like measuring how far to tighten the nut when doing wheel bearings?

The supercharged Ford T-Bird had an electric vacuum pump and i would guess that the supercharged GM cars probably did too. So should be available in your better Pic-n-Pulls
Don

Electric Vehicle forums have a list of turbocharged cars that use a electric Vacuum Pump for the Brake Booster.
However, their performance is in the 17inches of Hg range.

I took the list to the Junk yard and did not find even one.

You can buy them new for around $250 more or less.

I actually bought a Thomas Vacuum Pump from eBay last year but never did anything with it.

Summit Racing sells one made for the job but again expensive.

Could someone cap the damn thing off in a manual car with no vac modulator, and run an electric pump for locks, power brakes, and CC (if applicable)? Added benefit that power brakes will still work with the motor off.

If I remember correctly funola was looking into something like this not so long ago. I'll try and find the link.

EDIT - here it is

http://www.peachparts.com/shopforum/diesel-discussion/284848-vacuum-pump-alternatives-6.html

This happened to a friend on a 617, but the valve was not damaged. The finger operating the valve is softer metal so it bent. No head removal. He replaced the finger, reapaired the vac. pump and was back in business.

I have a thread somewhere where I bought a late model Volvo 12 Volt Electric Vacuum Pump. I conneced it up to the main Vacuum Line and directly to the Battery.

I drove around with it and everthing that needed Vacuum worked normal.

I touched the Electric Vacuum Pump and Blisterd my Finger on it; it was so hot.
So with out the other do-dads like a Check Valve, Relay and Vacuum Switch to turn it on and off I think the Pump was on the way to burning up (this particular Pump seemed to be a Vane type Pump).

Unforunately the purposly made for the Job Vacuum Pumps like Summit Racing Sells are as expensive as a Mercedes Vacuum Pump.

I do not know if there would be any safety issues but I had been wondering if you wired a switch up to the Brake Pedal to turn the Vacuum Pump one only when the Brake is appled would work.
But, charging system problems seem to be more common than Vacuum Pump Failures so I guess it would be slightly less safe.

The above might be helped by adding a Vacuum reservoir just for the Brake Booster.

I am not sure if the locking system could be worked into that. If the Brake/Switch was wired directly to the Battery perhaps to work the locking system when the Car is turned off you could simply tromp on the Brakes a few time to charge up the reservoir of the locking System.

Unfortunately there is more than one way for the Vacuum Pump to be destroyed.

New Vacuum Pump Ruined due to too much Timer/Intermediate Shaft End Play (the title from my notes).
http://www.peachparts.com/shopforum/showthread.php?t=258062&highlight=vacuum

Even though this may seem like a great design, it is a good design, which means that it's predicated (my opinion) on the assumption that the car will probably be retired after 300,000 miles/500,000 km.

If you look at over-the-road trucks, which are made to be rebuilt after a million miles or so, they have a bolt-on vacuum pump for that reason.

When I get a DeSoto Suburban and redo it with a Perkins diesel engine like its Euro version was, I will use the external vacuum pump on the brake booster, for that reason. Building a truly bulletproof car is not hard, if you want to expend the extra cash.

On the older MBs, it's easier than you think.