TheSamba.com :: View topic - "Dual-circuit" Brake system????

Tue Jun 05, 2007 6:43 am

I've been wondering about this for years..... I'm sure we have all seen the digrams in the factory manual and various other repair manuals about the "dual-circuit" brake system our volksies are *supposed* to have......with each chamber of the Master cylinder operating one front and one, opposing, rear brake cylinder....but, I have never, ever actually seen a VW piped like this.....it's always front chamber to the fronts and the rear chamber to the rear......

Was this an Idea that was dropped....or never made it to production....or has ever VW I've every seen been modified?

Tue Jun 05, 2007 7:04 am

Dual circuit brake systems came out in the late-1960's. I am certain someone will post what year the changeover occurred, but yes, VW's were plumbed this way and an earlier car just needs the master cylinder to be a dual-circuit vehicle.

Tue Jun 05, 2007 7:40 am

Hrmmmm....well, I've had a couple of '74's (Thing and Std. Beetle) and a '72 bus as well as a 79 bus....as far as I can recall, there are only 3 fittings on the master cylinder for the brake lines.....with the two front ones fitting on the front end of the MC and a single pipe connecting to the rear chamber that splits to the two rear wheels.........seems the MC would need 4 fittings to even make this system possible....

Tue Jun 05, 2007 8:25 am

VW's since 1967 have a dual circuit braking system not a "Dual Diagonal Circuit" system. VW's operate both front brakes on one circuit and both rears on the other.

Tue Jun 05, 2007 10:21 am

Why would you ever want a dual circuit system to be split right side, left side instead of front circuit, rear circuit? If you lost the right side circuit and only had left side brakes, that sounds kinda dangerous to me. Every brake system (including non-VW) I have worked on has been spilt front/rear. I only have two connection points on my VW dual circuit master, one for the front brakes and one for the rear.

Tue Jun 05, 2007 10:29 am

Vedauwoo wrote: Hrmmmm....well, I've had a couple of '74's (Thing and Std. Beetle) and a '72 bus as well as a 79 bus....as far as I can recall, there are only 3 fittings on the master cylinder for the brake lines.....with the two front ones fitting on the front end of the MC and a single pipe connecting to the rear chamber that splits to the two rear wheels.........seems the MC would need 4 fittings to even make this system possible....

Hence the dual circuit. Two front brakes are served by one line, and the rear by another line. Where did you come up with a single circuit?

Tue Jun 05, 2007 11:24 am

A dual circuit master cyl has two pressure chambers inside. It does not matter how many outlet lines come from each chamber (you do need at least one though :lol: ) you will get even pressure in each line. So two separate lines to the front are equal to one line run to a tee and then split into two lines. The volume of fliud each chamber moves (the chambers are different sizes) determines the braking bias front to rear. The larger volume chamber will privide more force which is why it goes to the front brakes.

Tue Jun 05, 2007 11:56 am

Icy wrote: Vedauwoo wrote: Hrmmmm....well, I've had a couple of '74's (Thing and Std. Beetle) and a '72 bus as well as a 79 bus....as far as I can recall, there are only 3 fittings on the master cylinder for the brake lines.....with the two front ones fitting on the front end of the MC and a single pipe connecting to the rear chamber that splits to the two rear wheels.........seems the MC would need 4 fittings to even make this system possible....

Hence the dual circuit. Two front brakes are served by one line, and the rear by another line. Where did you come up with a single circuit?

Not a single circuit...but a dual that is different than what is in my manual.....even as late as my 1974 Beetler manual details that the brake cylinder is a front left with a rear right on one chamber and a front right with a rear left on the other......I guess they just never bothered updating the graphic.....and then of course there are statements in things like the "Muir book" that were never changed , despite revisions through the years.

Not complaining or anything....I was just curious why mine had been different than what was shown in the manuals....I gather now, that a change was made to what I have always had in 1967.

Tue Jun 05, 2007 12:08 pm

I have an old Bentley Manual for the 70-78 Beetle, SuperBeetle and KG. In the brake section it only refers to a front and a rear brake circuit, nothing mentioned about mixed front to back circuits or criss-crossed brake lines.

Fri Jun 08, 2007 2:15 pm

When the two front wheels are on the same circuit (and the two back wheels are on a different circuit), it is called an "II-split" brake system.

When the right-front and left-rear wheels share a circuit (and the left-fron and right-rear wheels share a different circuit), it is called an "X-split" brake system.

Volkswagen began using dual circuit master cylinders in the mid- to late-sixties. I believe that all air-cooled Volkswagen brake systems are the "II-split" type.