where is my brake fluid going?

[fatmobile]

So there is an o-ring that seals the master cylinder to the booster on MK2s?

 On the later Rabbit there is a seal inside the booster that seals when the peddle is up.

[92EcoDiesel Jetta]

So there is an o-ring that seals the master cylinder to the booster on MK2s?

 On the later Rabbit there is a seal inside the booster that seals when the peddle is up.

Yep a pricey one too. p/n from the local dealer 357611243  $11

[92EcoDiesel Jetta]

ATE MC arrived. It was on sale for $100. It's made in the Czech republic and the body is Aluminum. Came with the $11 seal between MC and booster and 2 new mounting nuts too. Hope it's a good one and the last one I have to buy.

[92EcoDiesel Jetta]

Checked the identity of my caliper per this guide http://www.ate-brakes.com/www/download/ate_de_en/themes/qualitaet_und_tradition_en/download/pdf_documentation_tecidentify_en.pdf and it is valid (not counterfeit).

The MC is not marked where it's made, just the box label with the codes.

[92EcoDiesel Jetta]

The ATE MC is in. Time will tell if it leaks or not (hope not). I did look at the pedal end of the booster and there is signs of brake fluid drip. It was just a tiny spot where it is dripping from and not a mess at all. The carpeting is not wet but it appears it is dripping on this thick foam sound insulation with fiber backing and the brake fluid soaked into the foam. The layers are foam/ fiber backing/ carpet. I think the fiber backing may be preventing the carpet from getting wet. When I get time, I will pull the carpet back to have a better look. The good news is all that brake fluid did not go into the engine.

[92EcoDiesel Jetta]

So there is an o-ring that seals the master cylinder to the booster on MK2s?

 On the later Rabbit there is a seal inside the booster that seals when the peddle is up.

I thought all brake boosters has an o-ring to seal the vacuum side of the booster against the master cylinder. How are the later Rabbits different? Do you have a diagram?

[fatmobile]

No, I don't have a picture either.
Autohaus shows one for an '82 with no place for an o-ring.
 http://www.autohausaz.com/search/product.aspx?sid=oxdw4eyohogjl4zmzo3yerq0&makeid=800026@VW&modelid=1369942@RABBIT%20&year=1982&cid=14@Brake%20%26%20Wheel%20Hub&gid=1836@Brake%20Master%20Cylinder

[92EcoDiesel Jetta]

No, I don't have a picture either.
Autohaus shows one for an '82 with no place for an o-ring.
 http://www.autohausaz.com/search/product.aspx?sid=oxdw4eyohogjl4zmzo3yerq0&makeid=800026@VW&modelid=1369942@RABBIT%20&year=1982&cid=14@Brake%20%26%20Wheel%20Hub&gid=1836@Brake%20Master%20Cylinder

Just looked in my 77 to 82 Jetta/ Rabbit manual and it shows 2 types of brake MC (w and w/o brake servos). The ones w/o servos has no o-ring (makes sense), ones w servos has what's called a Vacuum seal O-ring. Hard to tell from the MC pic of what linked to if there is a groove for an O-ring or not. My point is that there must be some kind of a vacuum seal (O-ring or gasket) between the MC and brake booster.

[fatmobile]

There is.
 It's inside the booster.
 I think I posted that in this thread once already.
Any master cylinder leaks go down the front of the booster.
I think you can find more than 2 types of masters for the Rabbit.
 Non-power and 2 types with power brakes.

[92EcoDiesel Jetta]

There is.
 It's inside the booster.
 I think I posted that in this thread once already.
Any master cylinder leaks go down the front of the booster.
I think you can find more than 2 types of masters for the Rabbit.
 Non-power and 2 types with power brakes.

Hard to tell what you are trying to describe. "There is" meant there is an o-ring or there is a groove in the MC that you linked to?

I think you meant an o-ring, which is inside the booster (by the way, I have never seen a booster like that). If that is what you meant, then the booster must have an inverted cup where the o-ring seals against the moving shaft of the MC, rather than an o-ring seal against the body of the MC.  Were you also saying such a MC would leak brake fluid down the front of the booster only?

[fatmobile]

There is.
 It's inside the booster.
 I think I posted that in this thread once already.
Any master cylinder leaks go down the front of the booster.
I think you can find more than 2 types of masters for the Rabbit.
 Non-power and 2 types with power brakes.

Hard to tell what you are trying to describe. "There is" meant there is an o-ring or there is a groove in the MC that you linked to?

I think you meant an o-ring, which is inside the booster (by the way, I have never seen a booster like that). If that is what you meant, then the booster must have an inverted cup where the o-ring seals against the moving shaft of the MC, rather than an o-ring seal against the body of the MC.  Were you also saying such a MC would leak brake fluid down the front of the booster only?

"There is" is in response to your post; "My point is that there must be some kind of a vacuum seal (O-ring or gasket) between the MC and brake booster.
"
 I actually should have posted "there isn't" that it isn't between the master and booster. It's inside the booster. Seals from the inside.
Nothing between the booster and master to seal it,.. if it leaks it goes down the front of the booster.

[92EcoDiesel Jetta]

There is.
 It's inside the booster.
 I think I posted that in this thread once already.
Any master cylinder leaks go down the front of the booster.
I think you can find more than 2 types of masters for the Rabbit.
 Non-power and 2 types with power brakes.

Hard to tell what you are trying to describe. "There is" meant there is an o-ring or there is a groove in the MC that you linked to?

I think you meant an o-ring, which is inside the booster (by the way, I have never seen a booster like that). If that is what you meant, then the booster must have an inverted cup where the o-ring seals against the moving shaft of the MC, rather than an o-ring seal against the body of the MC.  Were you also saying such a MC would leak brake fluid down the front of the booster only?

"There is" is in response to your post; "My point is that there must be some kind of a vacuum seal (O-ring or gasket) between the MC and brake booster.
"
 I actually should have posted "there isn't" that it isn't between the master and booster. It's inside the booster. Seals from the inside.
Nothing between the booster and master to seal it,.. if it leaks it goes down the front of the booster.

I've been trying to visualize how a booster is constructed as you described and came up empty. Maybe I am just dumb. Can you help explain? Maybe post a diagram or a year and model where such a booster is used? Is the MC for such a booster made differently?

Here is a great video showing how a booster and MC work the way I am familiar with.
https://www.youtube.com/watch?v=bGKJOICWmFQ

You can see the o-ring that seals MC to booster briefly at 0:33 to 0:36 and 1:46 to 1:55 secs. At 2:22 secs. there is a diagram showing the complete system (they left out the o-ring in that diagram).

Can you tell me where the inside booster seal is (in the diagram at 2:22 secs) in the system you described and how it is constructed? The diaphragm has to move in order to push on the hydraulic rod and the vacuum side of the diaphragm cannot have any vacuum leaks otherwise there is no vacuum assist.

[fatmobile]

The one I linked to at autohaus shows the difference,... no groove for the o-ring.
 I can't show you the difference in the booster.
 I wish I knew you wanted this info when I changed the master on the '82 Rabbit a couple months ago,
 I'd have taken pictures.
 Maybe if you can find a pic of the '82 booster is will show you how it's sealed.

[92EcoDiesel Jetta]

Does your booster look like this http://www.partsgeek.com/7vtbltt-volkswagen-rabbit-brake-booster.html?utm_source=shoppingcom&utm_medium=pf&utm_content=dc&utm_campaign=PartsGeek+ShoppingCom&fp=pp&utm_term=Volkswagen+Brake+Booster ?

Here's what fcpeuro has on an 82 Rabbit master cylinder https://www.fcpeuro.com/Volkswagen-parts/Rabbit?year=1982&e=961&m=20&keywords=master%20cylinder

There is a grommet which serves the same function as the o-ring. Did you replace the grommet? How does it go in? I assume the small diameter end goes into the MC and the large end goes in the booster? If so there must be a drain of some sort in the grommet for any MC fluid leaks to drain out the front of the booster? What I do not understand is how it could drain out the front of the booster only and still provide a vacuum seal.

The one I linked to at autohaus shows the difference,... no groove for the o-ring.
 I can't show you the difference in the booster.
 I wish I knew you wanted this info when I changed the master on the '82 Rabbit a couple months ago,
 I'd have taken pictures.
 Maybe if you can find a pic of the '82 booster is will show you how it's sealed.

[fatmobile]

When you say "it" could drain from the booster what do you mean?

 I believe that grommet is to seal around the reservoir,.. so yes I used it. Of course the rebuilt master cylinder had it's own grommet but I've heard of them leaking and that was the case with the new grommets on my rebuilt master,... so I reused the ones from the old master.

  The top link to the partsjerk booster shows you real well how the seal looks inside the booster and it even has a little divot for the brake fluid to leak down the front of the booster if the master starts leaking. Kind of hard to get an o-ring to seal with a deep dent where it rides.
 Oh if you send a postal money order to partsjerk they will wait 3 weeks before sending your part,... that info isn't on their website.