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Discussion Starter #1
I'm in the middle of a rebuild of my brakes on my 71 Spider. I've just done the calipers and I'm about ready to install a rebuild master cylinder. The vacuum booster looks good - no fluid or signs of corosion inside, and the rubber valve looks good.

When I took off the old master cylinder I noticed that there is no rubber o-ring or gasket between the master cylinder and the booster. Should there be one? There's no sign of one on the car and the rebuilt MC I bought didn't come with one.

Can't seem to find anything conclusive on the Cardisc.


Thanks,

Lokki
 

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Trained (ex)Professional, , 1953-2018 RIP
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16,232 Posts
Is there a groove machined into face of the pushrod end of the master? If so, then it takes a square-cut o-ring about 35-40mm in diameter (for a 22mm ATE M/C anyway). Cant say if a 20/22mm Bonaldi/Benditalia or a 20mm ATE use the same size but I'll send you a dozen of the 22mm ATEs if you'd like.
 

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Discussion Starter #3
Papajam - thanks for coming to my rescue, as usual. There IS a groove machined into the replacement master cylinder, and there was (well sort of is) an o-ring on the original one. It was just so flattened and dirty I just didn't see it when I pulled the old MC off. It looks like it's been in there for 40 years or so...

It seems to be square cut and I'd say it's about 47-48 mm OD (1 7/8 in) by my best measurement.

Rebuilt master cylinder 71 Alfa 20 mm.JPG
Machined groove

Rebuilt Master Cylinder 71 Alfa p2.JPG
20mm ATE

It doesn't seem as if it'd be the same as the ones that you have, but it were, I'd appreciate one.

Thanks,

Lokki
 

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Trained (ex)Professional, , 1953-2018 RIP
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16,232 Posts
Well, this confirms it; my memory is gone.
The M/C O-rings I have are round, not square. And my guess on the size was a bit off as well. They measure (about) 42 x 47 x 2.5mm.
Searched some more and managed to find one new square cut O-ring that is dimensionally identical to the round ones. I'll get a package off to you on the AM.
 

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Discussion Starter #5 (Edited)
Papajam - Thanks! But you can save that stamp -
A little research on the internet came up with this:



397 Piece Metric O-Ring Assortment

A Harbor Freight metric O-ring assortment for $7.99

I went out and bought one - the proper size is included in the assortment. They're nitrile rubber so they should be OK with brake fluid.

For future reference, in this kit the size of the o-ring that fits the ATE 20mm Master Cylinder is identified as #325.

I don't know if this is universal or particular to this kit, but it's a perfect fit.

I now have enough 0-rings to start a business, but I also have 10 of the one size I need. Not bad for $7.99.
 

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Trained (ex)Professional, , 1953-2018 RIP
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16,232 Posts
The 325 o-ring is an industry trade size number. It's standard size is 1-7/8"OD x 1-1/2"ID x 3/16"Dia. This crosses to 47.6 x 38 x 4.8mm. Not exactly the same dimensions as the 22mm M/C but I don't know if the 20mm M/C o-ring is dimensionally the same.

But more importantly, please read this rubber materials compatibility information. Nitrile does not bode well with glycol based brakefluid. By looking at the nitrile compatible fluids, petroleum products among others, it would be a fair wager that the o-rings in your kit would swell up like balloons if exposed to glycol based brakefluid. A simple test would be to put an o-ring in a dish of brakefluid and measure the dimensions in a week or two (if a couple of weeks seems long, consider that I tested a piece of visually correct brakefluid reservoir hose for two YEARS before I deemed it safe for use in a brake system).
On the other hand, and in theory, the o-ring should not come into contact with any fluid anyway. It's purpose is to seal the front portion of the booster from atmosphere.

The o-ring package was mailed to you this AM by my wife while I was work.
 

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Discussion Starter #7 (Edited)
Thank you again Papajam - as usual the Devil seems to be in the details. The VW people weren't aware of the problem apparently, and I made the mistake of assuming that 'safe for hydraulic fluid' equaled safe for brake fluid. Guess not.

Anyhow the O ring is greatly appreciated.

Lokki
 
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