blown vacuum pump and a couple questions - PeachParts Mercedes-Benz Forum

pselaphid · #1 09-09-2007, 12:27 PM

Hey folks, I am embarking on a 617 vacuum pump rebuild. Mine went out suddenly on a lengthy excursion Fri. so I spend a nervous couple hours driving without power brakes. Anyway, I have the DYI pages in hand but have one question that seems not to be spelled out. Searched forum and can't immediately find an answer among the 500 or so "vacuum pump" results.

Regarding the little blocks that are used to compress the springs (?though the notes say "relieve the pressure") while the case is mounted on the homemade frame, what is the height of the blocks? Those who have done this should know what I mean, but I'm attaching the relevant image. I haven't removed the pump yet so don't have a complete visual feel for what's involved. The DYI notes don't make it look too intimidating, but I'd appreciate any supplemental tips from those who have done it and can offer tips or warnings. Nothing catastrophic seems to have happened to indicate that any metallic damage is involved, but of course I'll check everything on reassembly.

Here's what happened. Got off interstate, noticed hard brake pedal--no power brakes. Car won't shut off without pushing the IP stop lever. Center vents won't open with AC on, no central locks. Different, and wrong trans shifting points. No broken or detached vacuum lines. Blown vacuum pump, yes? One other malady that also is coincident with all this is that engine stumbles and stalls a time or two on startup. Started and ran smoothly prior. Related issue???

Thanks.

tangofox007 · #2 09-09-2007, 01:54 PM

Quote:

Originally Posted by pselaphid

Regarding the little blocks that are used to compress the springs (?though the notes say "relieve the pressure") while the case is mounted on the homemade frame, what is the height of the blocks?

Around 3/4" should do. When I overhauled my pump, I was able to get the piston nut started by holding the back of the pump against a flat surface, but I would not recommend that, as it would be easy to strip the first couple of threads. As you will see, the piston rod is spring loaded to the down position, so the spring tension must be overcome to extend the rod far enough to get the threaded end through the piston.

I would suggest that you inspect the check valve at the pump before you condemn the pump itself. It is inside the nipple where the hard lind attaches to the pump. You can remove it with the pump still installed. If it breaks internally, it can manifest the same symptoms as a failed pump.

pselaphid · #3 09-09-2007, 03:47 PM

Hmm, I just took the metal line off, including the fitting threaded onto the pump that it screws into. I see no check valve. There is one further up the line near the brake booster, but I think that's not the one you are referring to (and it works anyway). If I peer into the pump I can see what appears to be a flap against the opening. Maybe mine is different and has an internal check valve, or I'm missing the obvious? Anyway, I'll wait a while then will probably proceed with the removal. Thanks for the tip on the length of the wood block, that's helpful.

tangofox007 · #4 09-09-2007, 04:13 PM

The check valve is part of the nipple. It should permit air flow in the direction of the pump only. Sometimes the retainer breaks and the check valve components fall into the pump and block the airflow. Make sure that the "flap" that you see is not part of the check valve.

pselaphid · #5 09-09-2007, 04:34 PM

Well, upon probing with a pair of forceps, the "flap" just turned out to be the other side of the entry bore below the fitting. I can confirm that if there ever was a check valve there, it and all traces of it are gone. If it was there, its now in the pump. But I don't see a trace of a broken retainer, valve body, or anything, just round compression fittings with a metal tube between them. I don't think it ever had a check valve at that particular location, but I'm still curious about why or why not.

OK, its coming out.

blindwolf · #6 09-09-2007, 08:44 PM

When I rebuilt the vac pump on my 83 300d, I used two pieces of 1x2 to force the piston as the pujmp was pulled down on the mount board. A digital camera always comes in handy when taking an assembly apart for the first time. You will also need a heat gun to replace the piston without damaging the new skirt. About ten minutes of heat replaces a couple hours of sweat. LOL

pselaphid · #7 09-09-2007, 10:54 PM

Thanks, the camera is sitting next to the instructions ready for action. Did your's have a check valve as mentioned previously?

charmalu · #8 09-10-2007, 12:32 AM

On e-bay there is a brand new pump for $269.00 free shipping.

about the cheapest I`ve seen. type "Vacuum" in the search.

Charlie

blindwolf · #9 09-10-2007, 07:57 AM

My pump only has the three check valves in the pump body.

tangofox007 · #10 09-10-2007, 09:44 AM

The service manual actually refers to the nipple check valve as an "oil retaining valve." It prevents the brake booster from sucking oil out of the vacuum pump while the engine is not operating.

The vacuum pump itself will work fine without the valve, except when its broken components obstruct air flow into the pump.

pselaphid · #11 09-11-2007, 09:10 PM

And an apology to tangofox007 for ever doubting that I had an oil check valve. First thing that happened when I took the pump off was the little spring fell out, then I found the other piece lodged against the intake diaphragm. Right on the money with that one though I'm still mystified by how the thing was installed in the first place. I'm not sure I could have fished it out without removing the pump and I had the kit anyway so a good excuse for a rebuild. The reason I'm bumping this to the front, I hope, is that I took a few good pictures along the way and will post them if anyone is interested. Yes, no, maybe? Probably they are better than the ones on the DIY link.