bob p

I'm doing an original master cylinder replacement and the square-cut o-ring that goes on the master cylinder hasn't been included with the replacement master cylinder that I just bought. This is the sealing ring that forms the seal where the master cylinder inserts into the body of the brake booster.

Looking at the technical service manuals for the cars, all of the master cylinder + booster combinations use an o-ring to provide a vacuum-tight seal when the master is bolted onto the booster. Typically these are square-cut o-rings rather than round o-rings.

Looking at the technical specs for replacement master cylinders, it appears that many of the "rebuilt" master cylinders from suppliers like Raybestos and Dorman don't include the seals. Some models seem to include them, others don't, and even the models that seem to have them in the photos don't seem to have them when you actually open the master cylinder in the shipping box.

So what's the story on the master cylinder to booster seals? It looks like they're a disposable part that should be replaced with the master cylinder. On the masters that I've removed, they sure look decrepit and spent, but on the replacement master that I bought, they're not even there.

Are they supposed to come with the replacement master cylinders? Or are you supposed to buy them elsewhere? Believe it or not, square-cut o-rings in the size that we need for these master cylinder applications are pretty difficult to find. Where do you get them?

Octania

I reckon, in a pinch, an o-ring that is in a stationary application could probably use a round section and still do what it needs to do just fine.

Making a problem where one does not exist?

If it's not sealing a sliding or rotation part.... why does the o-ring care what shape it is?

Don R.

Those O-rings are commonly missing or forgotten and the resulting lack a assist is immediately obvious. "Booster" Dewey has them, I made sure to buy a spare just in case!


http://boosterdeweyexchange.com/

bob p

This isn't anything about making a problem where none exist -- they make square-profile O-rings because there are situations that require them because round-profile O-rings will fail prematurely. O-ring failure can have disastrous consequences -- think about Morton-Thiokol and the space shuttle Challenger disaster... or just losing vacuum assist on your brakes because you used something that you thought was "close enough" for the job.

I started this thread because I've already tried a commonly available round O-ring and it's not going to be suitable -- partly because it's geometry is just wrong for the ring to properly seal the "gland", and partly because commonly available O-rings aren't made of the right material for extended life in the application. Of course, it's easy enough to just use the wrong device and kick the can down the road until you have another premature failure. But I'd rather take the time to assure that I have the proper engineering solution rather than an easy band-aid fix that isn't going to last.

If anyone really wants to understand how O-rings work, why different types are specified, how to size them, how to determine the proper width and depths to cut when machining O-ring channels, when to use different elastomers, etc., then I'd recommend taking the time to read the Parker Hannifin technical documentation about O-rings. Their "O-Ring Handbook" is the definitive on-line engineering reference on the subject, and it answers most of the questions that you can ever think of regarding O-ring applications.

Now that we're up to speed on why round O-rings aren't suitable replacements, the problem becomes one of sourcing the square-cut O-rings. You'd think that these things would come with the brake master cylinders, considering that they're shown in the product photos. But I didn't get them with my new master cylinder. This means that either the manufacturer failed to deliver a complete master cylinder that meets their advertised spec, or that I received a master cylinder that has had the O-ring removed from it, perhaps returned by a customer. (I plan to give the manufacturer a call on Monday.)

Replacing automotive square-cut O-rings for brake master cylinder/booster sealing is surprisingly hard. You can go to all of the big industrial suppliers of O-rings (like I did yesterday before I started this thread) and they're still not available. The proper size device just isn't in the product catalogs, which implies that the part that I'm looking for is a custom-made part specific to the application. Even so, these products seem to be "standard parts" in the automotive brake applications, so you'd think that they'd be easy to source.

An interesting question that comes up is why, if these products are "standard replacement items" in the automotive brake application, then why are they not being supplied on new master cylinders? Are the master cylinder manufacturers just cost-cutting and leaving us high and dry by not supplying the critical part? Or are these things listed in the GM parts catalogs, so that they're available for order at my local GM parts counter? Where does brakemaster Dewey get these things if they're not in the industrial supply catalogs? I'm sure he doesn't do enough booster rebuilding to qualify for a 10,000, 100,000 or 1,000,000-unit custom order. He has to be buying them in small quantities from someone.

My point in starting this thread is really about asking if I'm approaching this from the wrong end of the supply chain. This weekend I've already been though McMaster and all of the industrial manufacturers/suppliers listed in the Thomas Register, without having much luck in sourcing the desired part in small quantities. Obviously, what I need to learn is where these parts are commonly available to the end users. Are they available from the auto parts supply chains? From the Parts Dept at my local Chevy dealer? If so, I haven't been able to find them. I even looked on Booster Dewey's site before starting this thread, and his site doesn't even mention them. They must be an item that you can only get if you know enough to ask for it.

Since this is a small item that seems to be something that you can only get if you know who and where to ask for it, I thought I'd ask if anyone knows of local or regional sources so that I don't have to special order an O-ring from the other side of the continent. I'd rather not pay $40 to buy a pair of 69-cent parts.

Does anyone know if this part is in the GM Parts Catalog or the Olds Parts List? Does it have a number? If this is a custom part then I'm thinking that GM Parts has to be the best place to look for it, but to do that I need a Delco part number or a cross reference.

thanks.

Fun71

I found a couple sites in a quick search showing square cut o-rings for other make vehicles (Ford Thunderbird, BMW, VW). Perhaps you could use the measurements of yours to see if one of those fit?

Octania

oh damn school is in session

but seriously, I do appreciate the lesson set forth above.

there is 292 pgs of incredibly useful stuff like:

"Acid Resistant: Withstands the action of acids. "

you don't say!

bob p

Don, can you tell me the dimensions of those O-rings that you got from Dewey? He doesn't list them on his site.

Looking at your photo, I can't tell if their thickness/width is 1/8" or 3/16". It would also help to know the inner diameter and outer diameter, if you could take a look at your spare.

Thanks.

Charlie Jones

According to the Olds chassis parts book, in group 4.910 ;
there are two possibilities for the part you are looking for. Depending on which type of vacuum/master cylinder you have,

Gasket, master to vacuum cylinder 1960-63 ( Bendix) part # 3759031
and
Seal, master to vacuum cylinder 1960-63 exc. F-85 ( Moraine) part # 5456683

Now maybe you will understand why I told you that a Moraine master cylinder will not work on a Bendix vacuum booster .

bob p

Thanks for your help, Charlie.

By any chance would you have the exploded diagram that goes with the parts list? that would help a lot in terms of putting this all together.

Do you have the original parts documents, or are you working from scans that you obtained online? I'm hoping that you have the original book and that you did the scans yourself, and that it would be possible to re-scan the documents. I've tried very hard, but I just can not read the type on attachment that you posted, the resolution of the scan is way too low. I can't even see the part numbers that you listed, and there's no way to verify the numbers in your post are accurate. I'm hoping that a clearer scan is possible.

Thanks again.

Don R.

Bob, pm sent.

Charlie Jones

Bob, I scanned a complete 63 Olds parts book to my computer (1100 pages).
This websight only reproduces pictures in a rather small size.
You should be able to see them larger by upping the resolution of your browser. In the upper corner of the page should be three hashmarks, click on that and select zoom . Zoom to 200 percent and then go to the scan.
Failing that , save the image to "my pictures" and then you should be able to enlarge it there.

Here is the illustration of the power brake units both Bendix and Moraine.

bob p

Wow! 1100 pages! That'd be a huge job.

Thanks for posting those two exploded diagrams, Charlie. They're very helpful. They're also very nice sharp and clear scans. That parts list is still impossible for me to read, though. Enlarging it doesn't help, the resolution is just so bad on the attached image that enlarging it only turns small fuzzy numbers into large fuzzy numbers. If your copy is nice and clear, and the board is limiting the resolution of the image, would you consider sending a copy of that one page to me by email?

Charlie Jones

Send me your e-mail via PM

Don R.

Bob, here is a picture of my mc per your request.

bob p

Charlie, I've received the email with the 4 images. They're fantastic. Thanks for going to the trouble.