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Old 03-24-2003, 09:16 PM
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445
vacuum pump and brake booster 1982 300D Turbo

Several months ago I bought a 1982 300D turbo (W123). I've taken care of a host of minor problems but got stumped on the vacuum pump. Let me run through my findings and my suggested fixes and hopefully someone reading this can help:

After replacing a few pneumatic lock components I now have a leak free lock system (per my vacuum tester). The problem is that the brakes feel under power assisted and the door locks stop functioning after a few hours with the engine off.

I measured the vacuum pump as 17" Hg with the booster hose plugged and the rest of the hoses plugged. This isolates the system to just the pump. This seemed too low as I was expecting 20-23"Hg per my memory reading through other postings. With the brake booster conected the vacuum is only 11" Hg and the gauge needle is flickering. From this I decided both the vacuum pump and the booster needed work.

I bought a $50 rebuild kit for my pison air pump. I took the pump off the car and inspected the rocker bearing which looked perfect so I went ahead and rebuilt the pump with the 3 new check valves, pison seal and used all the other parts in the kit (see attached photo). After reinstalling the pump I now have no vacuum! This time instead of taking off the pump from the car I just took off the pump cover. It looks like the pison seal that sits over the o-ring sliped off the piston. I cleaned everything and put back the old (but undamaged) piston seal back on. Again 0" vacuum. I have now ordered a new (or rebuilt?) vacuum pump for just over $200. I've rebuilt many hydraulic and some pneumatic systems in the past and never had so much trouble/bad luck.

I assume things will be much better with the new vac pump except my wallet. Has anyone had a similar problem rebuilding one of these piston vacuum pumps?

I will probably still have a brake booster problem with a vacuum leak. Can the booster be repaired with new seals, grommets, etc? Does the entire booster ever ned to be replaced. How do I determine what needs replacing?

Thanks for your help,

Attached Thumbnails
vacuum pump and brake booster 1982 300D Turbo-piston-vacuum-pump-rebuild-kit.jpg  
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Old 03-27-2003, 07:55 PM
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445
Brake booster and vacuum pump

Sadly for me no one stepped in to give me any advise. Here is some follow up info and questions. I was unsuccessful rebuilding the vacuum pump as it was unable to pull a vacuum after reinstalling. Maybe i scored the center hole of the pison where the threaded rod comes through. It suprises me that the center hole is sealed with a metal (steel or was it aluminum?) washer topped with a nut, how air tight can that be? Instead I ended up paying $204 for a new one and I'm now getting 23" Hg instead of the 17" when i first tested the old pump isolated form the rest of the vacuum system.

My brake booster is also a problem as it is unable to hold any vacuum. I picked up a booster for $75 at a local MB salvage yard today. It tested OK in the yard with my Mityvac --no noticeable leak after 15 minutes of waiting. I had no idea if these things can be rebuilt and I can't afford to disable the car during the day as it is one of our daily drivers so I went for a used one.

Does anyone know if the o-ring between the master cylinder and the booster should be replaced? Also how should the inside of the booster be cleaned without damaging the diaphram. There is no obvious fluid but appears to have a light film of oil in there. I saw the salvage guy struggling under the dask to remove it and he simply cut the break lines to move the master cylinder out of the way. I guess i'll have to disconnect the brake lines to the master cyl to replace the booster? Any tricks to make this job easier/quicker?
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Old 03-27-2003, 08:27 PM
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i don't know of any tricks to make the swapout easier but the o ring you mentioned does need to have a good seal. if one of the used ones is still pliable it will probably be ok but seeing as how hard it would be to replace if it leaked i would opt for a new one. you checked the inline check valve on the booster line didn't you ?
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Old 03-28-2003, 10:19 AM
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Join Date: Apr 2000
Posts: 459
Ditto, if its hard it needs to be replaced, and if its pliable I would still use a little RTV sealant.

I have another question though. Does the vacuum pump have gaskets that sit between the block and vacuum pump and the vacuum pump cover and pump body? The cover I'm referring to is a piece that looks like its attached to the radiator side of the pump with hex bolts. Anyway, I have an oil leak coming right from that area, and am wondering how difficult it is to remove those parts (either the cover or the whole pump), just to replace any gaskets. If I have to somehow disassemble the pump then I will just live with the slight oil leak.


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Old 03-28-2003, 01:07 PM
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445
booster to master cylinder: The rubber o-ring is still pliable but I've never heard of using sealer on an o-ring.

Vacuum pump: Regarding your pump I can only tell you about mine. There is a thin (like paper) gasket between the block and the pump. You can buy it seperately and is also comes with the pump if you buy a new one. You can use a very thin coat of sealer on the block to pump gasket simply to hold it in place during the instal. Dont use RTV.

Between the pump cover and the pump body is a complex shaped o-ring. only use a light coat of oil on this one.

On my 82 300D I have to remove the radiator fan, upper hose, and PS belt to take the pump off (6 or 7 allen screws hold the pump to the block). To remove the pump cover (mine is held down with 6 or 7 slot head screws) I only had to remove the fan. The tricky part with removing/installing the pump cover "on the car" is to get all three pump check valves, their three individual gaskets and the cover's o-ring secured when reinstalling the cover. I used a small dab of grease to hold the cover o-ring in place.

As for your leak I would positively identify the location of the leak by wiping it dry and then looking where the leak propogates from. Then snug up the bolts (don't strip them as they go into aluminum) before contemplating digging in.
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Old 07-29-2005, 01:50 PM
Join Date: May 2005
Location: CALIF
Posts: 508
Vacuum Pump

1984 300d
Brake Master Cyl Leaking It Will Remove Paint On The Brake Booster
Replace Master Cyl $100.00 And Brake Booster $231.00 Remove Inside Brake 4 Mounting Bolts With Extension And Brake Pin, Fill Master
Cyl Before Install And Bleed Brake Cyl.
Low Vacuum Volume Is Pump, Replace With With New Pump $270.00
Brakes And Trans Shift Is Now Ok,

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