Hi,

Hope someone can help me, it's my first mercedes ('81 300SD).
(have wanted one of the old big ones since I was a kid

The vacuum pressure goes to 0 as soon as I step on the breaks.
I checked the pressure from the pump directly, it goes to about 15-16
after around 30 - 50 sec.
The system gets around 10-15, but everything drops to 0 as soon as I hit the breaks.

Do you thinks it's the Vacuum pump?
Or something with the powerbreaks?
Or am I lost here?

How can I test better?

I also get the standard problems of loosing the powerbreaks as soon as I step on them, and the engine has to be stopped manually (or wait for 1 min untill the pressure build up again.)

Your pump should be able to "pull" more vacuum.
Are you testing directly from the hard plastic line tha runs from the brake
assist to the vacuum pump? Take it loose from the master cylinder end +
hook up mighty vac ($25.00) for the plastic version @ autozone.I think the
vacuum pump is rebuildable.Tiny little valves sometime stick (spritz of brake
parts cleaner)Check with Phil @ the top of the page about parts.Vacuum at idlewill be different from some r.p.m.'s.

If your chassis does not have a separate pump for the door locks you may be
correct in your suspicion of the vacuum pump;and you might as well start
looking for a vacuum leak if the pump is O.K.

if you have a leaking master cylinder assist there is a sequence for "pump" testing it with your foot, I am unable to remember it at this time .

Some of the more experienced members will chime in I am sure!

Thanks for the reply, I'll get as close to the vac pump as possible to check how much it "pulls". Do you know what the normal (estimated) rate is at idle + how fast it build up?

I'm not sure about the extra pump for the doors, but it sound logical as they have never worked (not as high of a priority as breaks

The problem with loosing vacuum pressure started 2 months ago and this is the 4th time it acts up. The other 3 times it came back to normal after the car was parked for a while. This time I think it needs more than a rest.

thx

You need to get a MitiVac hand pump for best results.

It sounds as if you are not getting good vac from the vac pump, but a cracked plastic center piece (where all the vacuum lines attach) will do this, so will bad check valves at the pump, or just a loose line.

A bad booster will also dump all the vac when you step on the brake, I've had one that did this on a Dodge. Stalled the car when cold form excess air going down the intake.

You will hear it hissing around the brake pedal if the booster is leaking that badly.

Hook up the MitiVac and see how long it takes to develop vacuum -- if less than a minute, it's probably not the pump. If it takes a while, could be the pump or just a big leak -- attach the MitiVac in place of one of the small vac lines and try to pump down the booster, etc -- it will take a while, but you should be able to generate a decent vac. If you can't, something is leaking.

If it does pump down, but vents off instantly when you touch the brake pedal, the booster is shot.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I got a Vac meter and the pump seems strong and fine, 25 right away.
There is a slight hissing sound when stepping on the breaks, the pressure goes to 0 when pressing the breaks the first 1'. But when I hold the breaks down the pressure goes up again to 25. When releasing the breaks I loose some pressure again.

Any chance it's not the booster?

My manual did not have any info on changing it, just the master break cylinder... any ideas?

thx

Booster is definitely bad -- shouldn't loose that much vac that fast.

Booster is bolted in from the inside, and you have to take the pushrod off the brake pedal after removing the under dash panels. Not too familiar with the W126, but I believe you have to pull the headlight swtich knob off and remove the 1" nut underneath, the pry the surround off and remove some screws -- ditto on the keyswitch, pull the surround off and remove screws. Also some underneath. You will have to pull the carpet, should be some plastic "nuts" that hold the bottom of the black plastic panel on at the bottom, once those are off it should pull forward and down, then out.

You may only need to remove the black plastic panel and can leave the lower dash panel alone -- try that first, much easier.

Booster is held on by four nuts on the firewall, remove, then unclip the pushrod on the brake pedal, then the two nuts holding the master cylinder on (might want to do those first), then pull master cylinder away from booster and weasel the booster out. No need to remove brake lines from master cylinder, they are long enough and flexible enough to just pull it forward a bit.

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I'm with psfred. I just replaced the booster (twice - 2 bad ones) on the 300D with similar symptoms. Mine even started clunking on the 2-1 downshift. I could hear it leak at the booster when shutting the car down by pressing on the IP kill lever. I'm not sure I would risk flexing the brake lines -at least on a W123. I took my master cylinder off- it was leaking anyway and I replaced it.

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)

Great, thanks for all the help.

I'll give it a shot this weekend, got a used booster for $125.

My manual said to flush and change the break fluid when removing the master cylinder, is this recommended? (or could I just refill?).

Since you are going to introduce air into the system anyway and a good portion of the fluid is in the reservoir, do the flush.

If the master cylinder is old and you can, replace it while you are there.

Use recommended quality brake fluid type that is from a fresh and unopened container.

Nice peace if mind and often a firm pedal as well.

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)

Got the lower plastic panels out to take a look and guess I need to remove the whole dash panel (?).

Any help on how the dash panels are removed and if they come in segments would be really helpfull.