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.

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

Sounds like time to rebuild the vacuum pump.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

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!

__________________
Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive 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.

__________________
Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

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 ...".

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

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.

__________________
Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

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.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

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)

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!