Sounds like it is time to driver around a bit with a Vacuum Gauge connected so you can actually see if it is a loss of vacuum.

If not a vacuum loss then you could be having a brake booster issue.

Maybe the new check valve failed.

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84 300D, 82 Volvo 244Gl Diesel

Just to follow up with this. Did more invasive surgery and found the other part of the check value was still in the little tube. After I pulled it out, everything is back to normal.

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'80 300SD - '83 240D - '00 E55 AMG - '02 G500

Is this part NLA? What’s your plan for replacement? Or repair?

I had the same thing happen on my SD in 2015. I recall the valve was pretty expensive like $80 at the time.

I guess if I had to fix it today I’d probably put a valve in-line on the black hose.

ETA - I guess other distributors have it. Just did a search. Only NLA at Pelican.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

The last time I checked, vac repair parts were so expensive that it made sense to buy the entire pump at ~$300. Piersburg is the OE manufacturer and I bought one of those prior to needing my last pump for another car. Only the Mercedes pump and not piersburg was available the last time.

I have and am keeping 2 replaced pumps for parts. I also have one grabbed from a yard but no idea on whether it works.

I suggest loosening all of the bolts 1st after spraying with your favorite rust breaker because you don't want to go through the entire removal process only to be foiled by a couple of stripped bolt heads. Clean the bolt head, use a freshly squared allen socket and tap the socket into the bolt head before applying pressure to avoid stripping.

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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.

I'm not sure that outlet check valve is essential since I recall there is an internal outlet check valve (or two, forget) in series. Perhaps it is more of a belt and suspenders approach and/or improves efficiency. Every OM617 1-port vacuum pump I've seen in the junkyard had that outlet check-valve removed. Probably because it is an easy pull and I've seen people ask $100 for a used one on ebay even years ago, so they became golden. The rebuild kits are very hard to find now, except for the earlier 2-port pumps with a rubber diaphragm. Indeed, one of those might fit a later engine. You can source the internal check valves from rebuild kits for a VW Pierburg vac pump, but not the rubber head gasket or piston seal (teflon-leather).

Solutions for the future? Some here have installed an electric vac pump from a Euro car. Those are used for their brake boosters, so some worry about the constant usage of the transmission VCV in our cars wearing them out early. I haven't seen a reported failure yet. For the outlet check valve in question, you can buy inexpensive inline plastic vac check valves at auto parts, but intended for use in rubber hose, not the metal pipe and hard nylon vac runs in our cars. I filed away an alternator w/ integral vacuum pump from another diesel engine (Nissan truck? forget), but appears it might not clear the K-frame on our cars.

While you have the vac pump off, worry about the bearings on the follower arm. Many posts here about safer bearings with metal shields to contain the balls if the bearing fails. It failed in one of my cars to drag the arm on the ramp. One son drove it home ~100 miles with a strange rattling sound. It had worn the arm halfway in two. Another 100 miles and the arm would have come off to slide into the camshaft chain, which can cause engine destruction. Exquisite German engineering? An electric vac pump starts sounding wise.

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

I would lean to agree with you Bill. There are check valves in the pump. But my SD wouldn’t make consistent vacuum without the new outer valve. Perhaps my inner one leaked too. I had weird shifting, weak brakes and other symptoms.

I had exactly the same issue where the valve fell apart. I took off my pump and retrieved the pieces. I think I’d try JBwelding it together if this happened again. It fatigued at the little plate just like in the photos above. Or trying the inline check valve. It’s just getting too expensive to acquire that gold valve.

It’s easy to strip the vacuum pump socket caps. Make sure they’re really clean and you use a good wrench to pull them. I stripped out a socket screw on my crank pulley and used this poor man’s extractor on it to get it out. I’m sure this can be done with the vacuum pump screws too. Just use a smaller socket.

Just tap on a snug fitting socket over the screw head with a hammer and unscrew it. The shock of tapping helped release it I’m sure. The hardest part was getting my socket off the screw after I got it out.

ETA - yeah electric pumps may be the endgame for these cars. My friend who converts volvo 240’s to electric adds a KNF Neuberger diaphragm pump to his cars to power the booster. Turns on with a relay when he starts the car. I used a similar pump in a piece of industrial equipment that I sold. It would run 24/7. The diaphragms would die once every three years like a clock. I think our cars will see about 1/10 that duty so I’d expect over a decade with KNF Neuberger pumps. The OEM VW stuff is diaphragm style too but I don’t know how durable they are.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

I was replacing the bearings in my pump. That was the primary reason to go into it. It's actually a second pump I bought on eBay. It seems to work OK and the 3 check valves inside seem to be good.

I ordered a replacement external check valve from ********AZ. It's too expensive, but the car is worth it, I think.


The car has been down almost a month at this point, waiting on parts and for me to have time to work on it. It may be time to sell it once I get the vacuum pump working again.