I have a 1982 Mercedes Benz 300 SD which I use as spare car due the distance of home to work and vice versa.
I use this car very sporadically just to make sure that everything is in running condition.
On my last drive I noticed that the brake is a little hard as if it was not assisted by the booster. More exactly as when braking with the engine not running.
My question: can this problem be from booster or vacuum pump, or both?
Or can it be related with brake master cylinder?
Thanks.

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Nyck - TX - USA

Pump check valve?

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Unlikely the MC. Their problems are the piston seal leaking, which you recognize by the pedal sinking to the floor while pushing steadily. Step 1 is check your vacuum. Remove the 2nd rubber tube off the main line so you can measure closest to the booster. If OK (>15"Hg vacuum), repeat with that tube connected to the car and your gage tee'd in. If lower, you have an excessive leak in that circuit. If too low, start plugging off all vacuum consumers. If still low, measure right off the main vacuum tube with the booster capped off. If still low, the vacuum pump is the problem.

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

Measure the vacuum as BillGrissom said. The free rantal tools at the chain autopart places should have a combination vacuum and fuel pressure gauge or just a vacuum gauge.

If it is getting vacuum some vacuum but not enough it is highly possible the mentioned check valve.
Or you can unscrew the check valve and see if the parts are still intact. If it s gunky inside spray it out with WD-40 to clean it. (that worked on mine.)

There is also an O-ring between the Mastery Cylinder and the Brake Booster that when bad causes a vacuum leak there.

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84 300D, 82 Volvo 244Gl Diesel

[QUOTE=dieselbenz1;4148284]Pump check valve?[/QUOTE
The "valve" that you are mentioning is the one inside of the pump?
I had rebuilt pumps before and I remember to replace a check valve there. Is this the one?

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Nyck - TX - USA

This "check valve" is the one mounted at the vacuum pump?

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Nyck - TX - USA

Yes. It's in the nipple where the hard line attaches to the pump.
There is a retainer plate which tends to rust out and allow the check valve components to block airflow into the pump.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

the pieces of the check valve can be tricky to get out, if it falls apart. I managed with a magnetic pick-up tool (a cheap telescoping one that I had to bend into a curve, and then made an attachment for my shop-vac from part of a small funnel, in order to suck out the plastic piece.

The replacement check valve was surprisingly pricey; I think the lowest price I could find was online-about $80.-

Yes, used vac check valves are priced $100 on ebay. Almost every 1982-85 300D I've seen in the junkyard had that check valve picked. If not, of course I grabbed it. The vacuum pump rebuild kits run ~$200 now. I rebuilt one of my pumps back when the kit was $80. The earlier two-tube vacuum pumps have cheap rebuild kits ($10) and they will fit later cars. Kits for vintage VW diesel vacuum pumps are cheap and a source for the little internal check-valve disks, but they don't have the piston-ring seal (Teflon & O-ring) which is often the failed part.

If you give up on the factory Pierburg vacuum pump due to costs, other options are an electric vacuum pump from newer Euro diesels (VW, Audi), which they use for their brake booster. But you must add a vacuum switch. Some say they might cycle too much in our cars, due to the constant demand by the VCV valve. A few posts here. Another thought is an alternator with an integral vacuum pump on the rear, but looks like it might bump into the motor bracket.

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

Yes.

If you pull it off and all the parts are intact you likely don't need to replace it. Clean it out with WD-40.

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84 300D, 82 Volvo 244Gl Diesel

I have not seen a vacuum pump rebuild kit that included a new check valve.

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84 300D, 82 Volvo 244Gl Diesel