Originally Posted by vstech

the transmission bleed is through a VERY small orifice, and it's only bleeding at wide open throttle...

Originally Posted by cbjukraine

so what about the VCV on the IP? It bleeds vacuum every time the transmission shifts, no?

Originally Posted by qwerty

Not true.



No. The VCV bleeds vacuum whether the transmission shifts or not. The "bleed" is a function of throttle position and is completely unrelated to actual shifts.

Originally Posted by Beagle

Déjà Vu! Lately there has been yet another spate of Vacuum Pump horror stories and angst that could so easily be avoided. Vacuum Pump failures are invariably caused by failure of the cam follower bearings – the cages wear through and disintegrate releasing the balls, surely not too surprising after over 30 years of continuous service?

I copy below a post I first posted here 10 years ago, my 2nd or 3rd I believe? Surprisingly after all the countless threads here on vacuum pump failures I am not aware of a single poster, apart from myself, having replaced the pump bearings or even suggested it.

For an investment of about 2 hours work and around $30 you can install new bearings and thereby buy yourself another 10 years peace of mind! Surely a worthwhile insurance against incurring the expense and ordeal of an engine rebuild? For many here it would appear not.

For the first time in over 50 years I am no longer running an MB Diesel. My faithful 1982 300D has been retired after 934,000km on the original engine and it has never once let me down. I have treated myself to a new car with a little more style and comfort, not an MB or even a diesel – a German brand I would never dare mention here!

Since this will be my final post here may I wish you all the best with your MB Diesels. Adios

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VACUUM PUMP FAILURE! - Are you neglecting yours??

The Vacuum Pump is a vital component of your braking system and its sudden failure will lose you about 70% of your braking efficiency instantly. Rambo should cope but your wife or daughter certainly would not - your life can depend on it. It is naïve to expect this important mechanical device to perform reliably for over 30 years without any servicing or inspection.

There are three types of vac pump fitted to the 123 series diesels – the diaphragm type, pre 1979, has two pipe connections and exhausts into the air filter housing. The piston type fitted to later models has only one connection and exhausts directly into the crankcase behind the pump through a pair of plate valves. Both types are interchangeable, piston type for diaphragm and vice versa. They have a check valve incorporated in the vac line pipe fitting after the suction plate valve.

A twin lobed cam on the fuel pump timer reciprocates the piston (or diaphragm) via a rocker mechanism with a roller cam follower. The forward stroke exhausts air while the double spring return stroke draws the vacuum. The mechanism and piston sleeve are oil-spray lubricated from the timing chain. When >25” is reached the vacuum parks the piston forward against the springs keeping the rocker roller almost clear of the cam apex. As vacuum falls (e.g. at braking) the roller once again moves back to contact the cam and restore vacuum.

Thus the vac pump should operate for less than 5% of the time that the engine is running, 15 sec at idle to 25” at start up (i.e. FSM spec.) and less than 10 sec to top up after braking IF you have no leaks in the vacuum circuit and the plate valves are sealing perfectly! Any leaks in the system will cause the pump to run continuously and it is not designed to do so. It’s no surprise then that those pumps are going to wear out, fail prematurely, and cause excessive wear to the thrust face of the fuel pump timer bronze bushing. If you have any leaks in your lock system isolate it until fixed as this will cause the pump to work continuously and also reduce braking efficiency.

The most common failure is the rocker arm bearings – the bearing cage wears out and disintegrates releasing the balls (dirty oil is a factor here!) N.B. - Visual inspection of the bearings will not reveal whether the cages are paper-thin and about to fall apart! Balls and debris get caught in the chain and crushed through it pushing off the end plates and/or skipping teeth on sprockets.

By the time that you hear mechanical noise from the vac pump at least one of the bearings have already failed. While the pump may continue to function for a short time the cam is now rubbing on the rocker arm instead of the roller, tipping and jamming the piston. and the mechanism will soon disintegrate. The chain breaks followed by the camshaft and from there on it’s a train crash.

Servicing and Rebuilding

Several kits are/were available from MB. Prices are very approx.

1) Complete Piston Pump:- MA 000 230 13 65 $580
2) Diaphragm Kit:- MA 000 586 41 43 $45
3) Piston Seal kit (with piston):- MA 000 586 17 23 $187
4) Rocker Complete with Brg's:- MA 000 586 08 43 $300
5) Plate Valve Kit:- MA 000 586 40 43 $50

The vacuum pump is a very simple device that can be stripped cleaned and rebuilt by any average DIY’er. Replacement of the bearings and plate valves can restore your pump for around $30. You will have little or no warning if your pump fails and I would suggest anyone running the original pump, regardless of whether you have a problem now, should consider a rebuild. Just two to three hours work will reward you with at least another 10 years piece of mind! You will be astonished at how much better your brakes are. A good pump will restores vacuum to the booster as you are braking.

The rocker roller bearings are available from any specialist bearing store (choose a reputable brand! SKF etc. - they may have to order) and are easily replaced. Plate valves too are standard and available from pneumatics stores cheaper if you are prepared to search. The rocker and bearings are identical on both types of pump. They are a standard # 626 "Precision" deep groove journal bearing 19mmØ x 6 x 6 (6 balls) @ about $10 each. A parallel 6mm pin is a press fit through the bearings and roller and can be pressed (preferably) or driven out with a punch.

N.B. Use ONLY an open SIX ball "Precision" 626 (i.e. Grade 50 or better) without metal dust covers even if they have been removed. "Precision" bearings never have metal dust covers. The eight ball 626 is a light duty utility (junk) bearing and is intended for use in draw sliders casters etc. Care is needed to avoid damage to the new bearings when installing – support and do not side load the ball races when fitting!

Have a nice day - Beagle

Originally Posted by funola

Great last post Beagle! Especially the part about the vacuum pump operating part time, which I have never heard before. It would make sense for some VP's to be still operational at 300K+ miles. I have a piston type VP from my 83 240D which I will have to disassemble to study how it operates, maybe even rebuild.

Originally Posted by ohio diesel fan

all this talk here lately of vacuum pumps exploding has me scared. where is it? i bought the service manual cd from ebay and just went looking through it hoping it would lead me to it, but it is either named something else, or they didn't scan that section and put it on the cd.

Originally Posted by kerry

I'm curious as to how the spring is held compressed also.