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Residual line pressure for drum brakes

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VWCOOL 23 Windows of Awesome

5,158 posts · joined Jun 2003 ·

#551336
My rear brakes (four-wheel discs; Commodore callipers front, Nissan callipers rear, Kombi master cylinder) are dragging - enough to get the discs hot after a freeway drive while the fronts are stone, dead cold. Although appearing to be, it's not the handbrake dragging (cables are loose when at rest; brake callipers levers are in 'off' position); is there a residual pressure valve in a standard Beetle or Kombi master cylinder that keeps a few PSI in the rear brake line/circuit (for stock drum brakes)?


[ Edited on 1-11-2006 by VWCOOL ]
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hellbugged 48's and straight cuts

5,080 posts · joined Apr 2003 ·

#551339
bay kombis with drums all round had a residual pressure valve....mounts out the side of the master cylinder.

they are between the master and rear brake line, just look like a 17mm boss. 3 psi from memory.

i used them on my streeter with late master and early drums all round and also on the off road buggy when the forces of rear "knock off" due to the stresses associated, saw the rears returning too far and causing a long pedal:thumb

can grab piccie if my ramblings make no sense

[ Edited on 1-11-06 by dumone ]

VWCOOL 23 Windows of Awesome

5,158 posts · joined Jun 2003 ·

#551387
Thanks for that - the MC I am using is a disc/drum one with no obvious line pressure hardware (unless it is internal). These dragging brakes are a bit of a mystery to me as I can find no mention of any pressure/anti-knock-off feature in relation to VW disc-brake systems...
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vw54 Administrator Always Waiting 4 Friday

16,723 posts · joined Aug 2002 ·

#551388
the residual pressure is cause from the flexiable rubber line braking down on the inside, it acts like a one way valve

You need to replace all the flex lines with new ones
I read it on FACEBOOK so it must be true


http://forums.aussieveedubbers.com/viewtopic.php?tid=91278&page=1#pid848630


http://forums.aussieveedubbers.com/viewtopic.php?tid=63561&page=1#pid624099


http://forums.aussieveedubbers.com/viewtopic.php?tid=4832&page=1#pid452603


http://forums.aussieveedubbers.com/viewtopic.php?tid=60650&page=1#pid574409

VWCOOL 23 Windows of Awesome

5,158 posts · joined Jun 2003 ·

#551409
vw54 wrote:the residual pressure is cause from the flexiable rubber line braking down on the inside, it acts like a one way valve

You need to replace all the flex lines with new ones
yeah, I've heard of that happening but as mine are only a few years old and the system 'bled' easily, I fugured no problem... But yep, will look into it
Pay your debts, CxxT

1303Steve Scirocco Rare

8,650 posts · joined Aug 2002 ·

#551432
Hi

I never had any issues with my old bug when it had Golf callipers or the later Brembo ones, must be something else, do you have enough pedal free play.

Steve

xornge666x Wolfsburg Wizard

460 posts · joined Sep 2006 ·

#551445
test it by pumping the pedal a few times with the rear off the ground. If the wheels are hard to turn after the pedal is released, crack the bleeder screw, if it becomes easy, change the flex hoses. Is it both sides or just the one??

vw54 Administrator Always Waiting 4 Friday

16,723 posts · joined Aug 2002 ·

#551568
yeah pedal free play could also b the problem

However if you find one flex hose is crook change the LOT
I read it on FACEBOOK so it must be true


http://forums.aussieveedubbers.com/viewtopic.php?tid=91278&page=1#pid848630


http://forums.aussieveedubbers.com/viewtopic.php?tid=63561&page=1#pid624099


http://forums.aussieveedubbers.com/viewtopic.php?tid=4832&page=1#pid452603


http://forums.aussieveedubbers.com/viewtopic.php?tid=60650&page=1#pid574409

Old Dubber Wolfsburg Wizard

605 posts · joined Aug 2002 ·

#551586
the MC I am using is a disc/drum one with no obvious line pressure hardware
reread what "dumone" said!
It just looks like the fitting where the pipe attaches to the MC.
Take the fitting off and punch out the valve.
Grahame
visit my club site at www.vwdcqinc.org.au

Baja Wes V6 Baja Pioneer

3,119 posts · joined Aug 2002 ·

#551596
My Baja started as a weird 1300 deluxe german import with 4 wheel drums.

It had the ~17mm boss type residual pressure valves on the front and back lines. I pulled them off, punched the one-way ball valve out and put them back on.

Newer master cylinders don't have these, I am not sure if they have something internal instead?

[ Edited on 2-11-2006 by Baja Wes ]
Wes - www.offroadvw.net - 200HP Quad Cam V6 in a VW Baja - with climate control... 🙂
www.taylorcycles.com.au - My DH MTB racing brothers shop.

VWCOOL 23 Windows of Awesome

5,158 posts · joined Jun 2003 ·

#551788
Yesterday, I replaced all rubber lines, installed two new rebuilt callipers (due to the fact one of the lines snapped off at the calliper - grrr!) checked the disc run-out (a little on one side that may contribute to heat via drag against the brake pad) diassembled the master cylinder just to check there wasn't an internal valve or something that wasn't obvious from just looking at it, and swapped the front and rear circuits with each other, just as an experiment to see if the front rotors would get hot. They didn't.

Have been told about these Nissan and Ford callipers that 'they all do that'! but no-one ever notices


[ Edited on 3-11-2006 by VWCOOL ]
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VWCOOL 23 Windows of Awesome

5,158 posts · joined Jun 2003 ·

#551903
Yes. It seems it's normal these Nissan (and the similar-design Ford) callipers drag a little and heat up the disc. I was worried when I needn't have been.

I updated my 'big brakes' thread today too.

[ Edited on 3-11-2006 by VWCOOL ]
Pay your debts, CxxT