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Rear circuit for the front brakes?
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Wulfthang
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PostPosted: Fri Jul 31, 2020 9:13 pm    Post subject: Rear circuit for the front brakes? Reply with quote

I just recently read that the twin circuit master cylinders that many of us use, gets plumbed back to front and front to back. The front brakes get hooked up to the rear m/c circuit and the rear brakes get hooked up to the front m/c circuit. Something about the rear circuit has more pressure/volume for the front brakes. Is that right? What happens if it's hooked up backwards? Weird braking? Hmmmm!
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dirtkeeper
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PostPosted: Sat Aug 01, 2020 7:00 am    Post subject: Re: Rear circuit for the front brakes? Reply with quote

If your talking about reversing the lines from what is stock configuration , Never heard of that. I got disc In front and type 3 in rear and works great . I guess I would have to take a look but I am pretty sure stock is front to front and rear to rears.
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Wulfthang
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PostPosted: Sat Aug 01, 2020 3:32 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

dirtkeeper wrote:
If your talking about reversing the lines from what is stock configuration , Never heard of that. I got disc In front and type 3 in rear and works great . I guess I would have to take a look but I am pretty sure stock is front to front and rear to rears.


According to Jeff Hibbard's Baja Bugs front to rear and rear to front IS the stock system! I'm looking at it right now. Page 95, bottom on the center print column. In the stock VW Bug, "the rear M/C piston operates the front brakes and the front M/C piston operates the rear brakes". The reason given, is because the front brakes require more fluid action because of the larger size of the wheel cylinder and the rear piston on the master cylinder pumps more fluid than the front one.

I hooked mine up backwards from that because it seemed like "front to front", etc. like what you said. Except now I think it was wrong! Does anybody know for sure?
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jsturtlebuggy
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PostPosted: Sat Aug 01, 2020 3:55 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

On a stock VW Bug with a dual circuit master cylinder the front part is for the front brakes and rear part is for rear brakes.
The front part of master cylinder circuit has a larger volume then the rear circuit for the larger front wheel cylinders.
It is not a larger diameter, it is a little longer inside the master cylinder.

Now if you switch wheel cylinders with putting larger ones on the rear and smaller ones up front for brake bias on a off road car, there are people that say you need to switch up the lines on the master cylinder when you do that.
On the buggy that been in my family (inherited from parents) since 1970 when it was using a stock dual circuit master cylinder, it never had the lines changed. It work well.
One thing that was always done was the brakes were keep adjusted, never letting the pedal get further then half way down before adjusting the brake shoes.
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Wulfthang
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PostPosted: Sat Aug 01, 2020 4:56 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

So then the Hibbard book is wrong? Well, I guess that means that I have them hooked up right!
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jsturtlebuggy
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PostPosted: Sat Aug 01, 2020 8:16 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

Yes the book is incorrect. Jeff is not among the living, so he cannot correct it.
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Wulfthang
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PostPosted: Sat Aug 01, 2020 8:55 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

Thanks for the clarification. That saves me from having to dis/reassemble it all.
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dustymojave
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PostPosted: Mon Aug 03, 2020 5:23 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

jsturtlebuggy wrote:
Yes the book is incorrect. Jeff is not among the living, so he cannot correct it.


When Jeff "Revised" his book in the late 1980s, he asked me to proofread it for errors. That was one of them. But his "Editors" at HP publishing got in a rush and decided to pass up some of the corrections and just call it "good enough" and sent it to print. They never were corrected.

This picture is the dual circuit master cylinder in my '58 Baja. I updated the master in 2009.
Image may have been reduced in size. Click image to view fullscreen.


Note that in my Bug the line connected to the rear port passes through the Napoleon hat to head to the rear brakes. And the lines for the front are connected to the front of the master. 1 port for the rear line to go to the 'T' at the rear torsion housing, and 2 ports at the front of the master to go to the separate front wheels. If you wanted to switch the lines front to rear, you would have to block one of the front ports and put a 't' in the line from the back of the master cylinder.

I'm actually surprised I don't encounter more comments about that error (or any others) that's been in a VERY popular book for nearly 40 years. There was no such discussion in his original self-published book "The Complete Guide For Going Off Road - For VW Owners". So it wasn't wrong. I don't have a copy of the original HP book as I already had Jeff's original book and he told me they were pretty much the same. The 1st copy of the HP book I got was the printer's proof that I still have.

The rear piston in the dual master is larger OD than the front piston, which in theory makes for more piston area. But the rear piston is connected to the front piston with a rod that extends forward from the rear piston to the front (actually all one piece, not 2 separate pistons). That rod reduces the working area of the rear piston dramatically so that the "smaller" front piston actually has more working area than the "bigger" rear piston.

There is another error in there at the bottom of page 95 where it says that the rear piston has more stroke than the front. Tough to achieve more stroke with the rear piston when the front and rear pistons are both part of a single piece of metal. Not only Jeff, but Joe got that wrong above too.
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dustymojave
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PostPosted: Mon Aug 03, 2020 5:46 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

For another clearer picture of a dual circuit master cylinder and the plumbing for it, check on page 6 of this thread.
https://www.thesamba.com/vw/forum/viewtopic.php?t=717494

Image may have been reduced in size. Click image to view fullscreen.

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jsturtlebuggy
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PostPosted: Mon Aug 03, 2020 9:16 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

No I did not get it wrong, the front circuit stroke is 17.5mm and rear circuit stroke is 11.5mm.
This is right out of of VWs "Without Guess Work" 1971-1974 spec manual.
Both front and circuits of master cylinder are 19mm in diameter for the stock one.
And front and rear pistons are separate, there is actually a spring and a few other small parts between them.
If you want to see what inside the master cylinder the Bentley Factory VW manuals have very good pictures of what is inside.
Spent close to 30 years working in VW repair shops and rebuilt many master cylinders and wheel cylinders before the insurance companies told shops that they have to install new hydraulic parts instead of rebuilding them.
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dustymojave
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PostPosted: Fri Aug 07, 2020 4:33 pm    Post subject: Re: Rear circuit for the front brakes? Reply with quote

Joe, I'm gonna have to say that you're probably right.

I've only had a couple of the dual circuit masters apart, and they both had 1-piece piston units with 2 pistons cast in 1 piece with no spring between. Each piston had a seal. I presume, based on what you're telling us, that the ones I worked with then were probably aftermarket replacement units that were not made the same as the OE units. Both of those had bad pitting in the bores that would not have honed out and got discarded without any attempt to rebuild.

I've rebuilt OTHER master cylinders, but those were the only VW dual circuit ones I've had open.
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