Vacuum
 

My mechanic says my PB unit (vacuum unit) is failing. Yet I have excellent brakes and at least short term in coasting say from 70mph with engine shut off I still get 6 brakes applications before I get the hard pedal. Yet when I attempt to stop the engine at idle, there is usually not enough to stop the engine unless I increase the engine speed. Are there two diaphrams in the unit, one for brakes and one for the rest of the demand.

Sounds like time to rebuild the vacuum pump.

Vacuum sucks. ;)

Sorry - had to say it. I'd check for any leaks in the shut off system - lines, diaphragm, etc. Diagnosing from a long distance I'd guess a small leak which vacuum LOVES to find so it can escape. You could try hooking up to the shut-off diaphragm with a known good length of hose and a hand-operated vacuum pump ( if you don't have one borrow the brake bleeder kit from an auto parts store - it'll have a pump in it). Pump it down and see if it holds vacuum. If not the diaphragm is probably shot. If it does hold vacuum try replacing the hose and see if that fixes it. If your brakes work well I'd doubt that the vacuum pump is bad.

Dan

I have a brand new out of the box vacuum pump on it!

[QUOTE=Dan Stokes;3597514]Vacuum sucks. ;)

Sorry - had to say it.

Well I do agree Dan. Vacuum sucks in more ways then one. But that is what we have and we will live with it in these great old 300D's. It could be the shut off is dumping it. I had a young lady riding with me the other day and she said she could hear air leaking, even thought maybe I was having a flat tire. My hearing is not good, I would never hear a small faint air leak.

If it is making a clacking noise, change it immediately. That happens when the ball bearings fail so that arm is dragging on "roller coaster ramp". In a few hundred miles, the arm will be cut thru and pieces slide down into the timing chain, most likely causing massive engine damage. I caught mine when the arm was cut half thru after ~100 miles driving by a family member. This failure can happen with no noticeable loss of vacuum.

Otherwise, do like Dan says in post #3. Step one is disconnect the main vac line from the pump and connect to just a vacuum gage. If you measure >15"Hg vaccum w/ engine idling, your pump sounds fine. If the shop didn't do so, run away and find a competent shop that can do obvious diagnostics before stating, "you need ...".

Thanks Guys. I was in a hurry when I wrote the first post, so let me add, before I installed the engine I installed a "brand new Pierburg vacuum pump. It is just running through its third 11 gallon fillup on that 240D tank, so it is brand new. But I forgot to mention I have the lock system and the vacuum reserve blocked off for now. So I will hook the whole system up and let you know how it performs. It is been six or seven years since I did some serious work on the vacuum system on my 85 and it is not something we mess with very much. And now I'm 85 and things do kind of get out of sight in a couple of years. So I will first hook it all up and then we will know where we are. Brakes and braking action is 100%, best I have ever had on a W123, no doubt about that.

Disconnect the main vac line coming from the pump and measure vacuum. Piersburg was an OE supplier and a new pump shouldn't be bad.

You can also try plugging the smaller lines coming off the main line and measure at one of the small lines while leaving the booster connected. You should have ~18' of vacuum. If not, measure directly at the pump. A booster or other leak may look like a bad pump and you want to be sure before spending a couple of Ben Franklins.

One of the competitors was way cheaper than other sites when I was in the market for a pump. Look around.

Is that noise the vac pump makes when going bad more obvious above the regular diesel clatter?

I've been around Diesels for a while and I find that I now sort of ignore the Diesel noises and can focus on unusual sounds that weren't there "before". I have no way to assess if this is common or not for Diesel owners.

Dan

I remember in the beginning being very concerned about certain sounds and then after time those sounds became normal ... if that answers your question.

Dkr.

Check the brake booster as per FSM 43-325

(Or get someone else to check it for you - print out the PDF)