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Location:
Castle Hill, Sydney
Registered:
February 2004
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rear brakes locking b4 fronts
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Sat, 17 July 2004 01:09
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i just put rear discs and bigger fronts into my t-18, along with a conrona master cylinder
the back brakes are locking up before the fronts
does anybody know how to resolve this or any clues how to?
other than softer pads in front/harder in back
help would be very appreciated! 
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Location:
Melbourne
Registered:
May 2002
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Re: rear brakes locking b4 fronts
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Sat, 17 July 2004 01:43
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Im not to certian on brake conversions for your car or how yours is setup, but obviously the rear brakes are smaller than the front, you will find that because of this they are easier to fill than the front. You could try the use of a bigger master cyl so the front gets enough fluid to fill up first. Or if that isnt the case employ the use of a a adjustable propotioning valve so you can adjust the rear bias to not lock first.
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Location:
Sydney
Registered:
June 2004
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Re: rear brakes locking b4 fronts
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Sat, 17 July 2004 03:16
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proportioning valve is the way to go... expensive if you go for in cabin ones but not too bad for standard ones... im going to put an in cabin one (better for burnouts  but i think they are not legal or something like that but i could be wrong (its something i heard in the wind a while back)
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Location:
Brisbane
Registered:
February 2003
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Re: rear brakes locking b4 fronts
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Sat, 17 July 2004 03:24
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try the proportioning valve that came with the corona brake setup?
the tech article on the RA23 f/disk upgrade also talks about modifying your proportioning valve to suit mod'ed setups.
adjustable valves are not allowed in the cabin - well as far as i know of.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: rear brakes locking b4 fronts
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Sat, 17 July 2004 04:39
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the reason backs lock up is because of the balance as other have said..
in hard braking, the weight transfer to the front of the car means that the fronts may be doing 70-80% of braking, and the rears only 30-20%.
this means that the disc x force of the rear brakes only needs to be (30/70) to (20/80), ie 25% to 43% of the disc size x force of the front brakes...
you can do a bit of math to work it out...
basically, work it out for one side.
disc size = either radius or diamter of the point of contact of the MIDDLE of the pad.
force = area of piston. (well, proportional to)
now do
(disc size) x (force) = Braking Force
(for want of better terms), for both front and rear disc..
then do the ratio BF(front) : BF(rear)
with proportioning valves, do they just reduce the flow of the brake fluid? or do they seperate the front and rear flow completely?
if the fluid line is continuous (even with a restrictor), when fluid flow stops, then the whole fluid system will have equal pressure, so i'm assuming that the proportioning valve decreases the flow initially to the rear, so that the rears don't lock up initially, as weight is transferred forward, and then when the car has balanced, more pressure is available to rear brakes?
anyways, Wilwood do a couple of bias adjusters which are pretty near. one has steps and the other is continuous adjustment..
and in cabin adjust is definitely not legal, but engine bay might be if your engineer says so...
anyhoo...
Cya, Stewart
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Location:
Rosanna, Melb
Registered:
June 2002
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Re: rear brakes locking b4 fronts
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Sun, 18 July 2004 12:15
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Always happy to help out anyone warming up a T18.
I am just repeating here what I have read elsewhere on Toymods, so search for the original page for more info.
Since the car came stock with rear drums there is a proportioning valve which does something funny (yes that is the technical term) to the F-R bias. When you change to disks you have to rip the guts out of it otherwise the rears lock up early.
The proportioning valve itself is the block that the two lines out of the master cylinder go into before heading off to each corner of the car.
So just check it out before you spend big dollars on some aftermarket bit.
Hen
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Location:
Castle Hill, Sydney
Registered:
February 2004
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Re: rear brakes locking b4 fronts
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Sun, 18 July 2004 12:26
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firstly thanks everyone for your help
yeah we've got a spare proportioning valve, pulled it apart and looks like there is no adjustment in it, so i have a couple choices
1- should i just try different sized springs in it?
or
2- does anybody know a toyota with an adjustable proportioning valve i can take from the wreckers that will bolt up?
thanks again
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Registered:
November 2002
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Re: rear brakes locking b4 fronts
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Sun, 18 July 2004 12:32
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Henn has it in one, but before I go on, you can change the bias in your standard prop. valve by cutting (say) two coils off the internal spring of your original valve and re-fitting that.
And back to Henn, yes rear disk brakes require different bias to rear drums and therefore your best set-up is to use a prop. valve from a car with similar sized rear disk brakes as standard.
If you really want an adj. prop valve, go and get one from a HQ Holden and fit that.
Mitch.
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