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Brake experts?



 
 
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  #21  
Old November 17th 04, 12:48 AM
Nate Nagel
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Dan Thomas wrote:

> Sounds like there's a high spot that's trapping air in the system. Are
> those wheel cylinders installed with the bleeder at the top so the
> cylinder fills up properly? Is, perhaps, the auto sitting facing
> downhill, with the master cylinder outlet lower than most of the
> cylinder, trapping air in it?
>
> The master cylinder cups should face cup-side forward.
>
> There's another phenomenon that likely isn't present here, but
> gives people fits: the shoes can hang up on the backing plates if
> there's no spot of lube on the rub pads, and they won't return to the
> anchor pin unless the car rolls along for awhile or goes over a bump
> or two. Adjusting the brakes forces the bottoms of the shoes out and
> they seem to drag a bit OK and there's good pedal, but after a few
> minutes on the road the pedal will go to the floor. No amount of
> bleeding will fix it. You have to bang the backing plates with a
> hammer to settle the shoes before adjusting the brakes.
>
> Dan


A-ha! just reminded me of something else that I didn't think to mention
(because it's a habit to me, working on old cars with drum brakes all
the time I just assume people know this stuff and you know what they say
about assuming...)

when adjusting drum brakes, one should adjust them *out* until the wheel
is locked and the star wheel will turn no farther. Then back the star
wheel off until the appropriate amount (light) of drag is felt at the
wheel. Alternately, if there is some reason you don't want to do this
(like you have self-adjusters which are a PITA to deal with) adjust
until the wheels drag lightly, then have a helper pump the brakes up,
then readjust, repeat as necessary. Reason being that the shoes may not
be perfectly centered so one shoe may be dragging while the other may
still be miles away from the drum.

If you work on the same car regularly, you probably will get a feel for
how the wheels should be adjusted without turning them, i.e. "seven
clicks backed off from tight" or something like that. Once you figure
out how many clicks to back the wheels off, you can actually do a more
precise adjustment than you can by turning the wheel, and do it quicker
as well.

good luck

nate

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  #22  
Old November 17th 04, 03:16 AM
G.R. Patterson III
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snh9728 wrote:
>
> I didn't mean an external leak; I meant a leaking piston seal. The
> piston moves forward when you push the pedal, but the fluid just leaks
> past it to the other side of the piston. The fluid is still in the
> master cylinder; it didn't squirt out into the world.


If that were the case, I would think that nothing would come out the bleeder
valve when I opened it up, but I do get some fluid out of each valve. About 25
pumps with a valve open will draw the level in the master down about half way.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.
  #23  
Old November 17th 04, 03:22 AM
G.R. Patterson III
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Dan Thomas wrote:
>
> Sounds like there's a high spot that's trapping air in the system. Are
> those wheel cylinders installed with the bleeder at the top so the
> cylinder fills up properly?


The way the backing plates and cylinders are designed, there's only one way they
can be mounted.

> Is, perhaps, the auto sitting facing
> downhill, with the master cylinder outlet lower than most of the
> cylinder, trapping air in it?


No, it's on the flat. It's possible that the master is mounted that way in the
car, but not enough to be obvious to the eye.

>
> The master cylinder cups should face cup-side forward.


Thanks. Joe Way sent me a schematic.

> There's another phenomenon that likely isn't present here, but
> gives people fits: the shoes can hang up on the backing plates if
> there's no spot of lube on the rub pads, and they won't return to the
> anchor pin unless the car rolls along for awhile or goes over a bump
> or two. Adjusting the brakes forces the bottoms of the shoes out and
> they seem to drag a bit OK and there's good pedal, but after a few
> minutes on the road the pedal will go to the floor. No amount of
> bleeding will fix it. You have to bang the backing plates with a
> hammer to settle the shoes before adjusting the brakes.


Doesn't sound possible on my car.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.
  #24  
Old November 17th 04, 03:31 AM
G.R. Patterson III
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Nate Nagel wrote:
>
> when adjusting drum brakes, one should adjust them *out* until the wheel
> is locked and the star wheel will turn no farther.


The adjustment mechanism on the '40 Ford has no star wheel. The bottom of the
shoe has a 1" hole in it. There is a brass eccentric cam that fits in this hole.
A keyed bolt fits through the hole in this cam. When the bolt is turned, the
bottom of the brake shoe moves in a circle. Up at the top of the shoe, there's
another eccentric cam, but this one just presses against the back of the shoe.

The adjustment procedure is to loosen the top cams and "play" with the bottom
ones until the bottoms of the shoes barely drag. The turn each one in the "up
and in" direction until they don't drag. Then adjust the top cams until the
shoes just drag.

With my new shoes and drums, the shoes are all just dragging slightly at some
point as you turn each wheel, but none are touching all the way around. That
will even out as they wear.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.
  #25  
Old November 17th 04, 03:26 PM
snh9728
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The difference is that with the bleeder valve open, you aren't
developing any pressure across the piston seal. With the valve closed,
you can develop pressure and the seal leaks.

Steve H

"G.R. Patterson III" wrote:
>
> snh9728 wrote:
> >
> > I didn't mean an external leak; I meant a leaking piston seal. The
> > piston moves forward when you push the pedal, but the fluid just leaks
> > past it to the other side of the piston. The fluid is still in the
> > master cylinder; it didn't squirt out into the world.

>
> If that were the case, I would think that nothing would come out the bleeder
> valve when I opened it up, but I do get some fluid out of each valve. About 25
> pumps with a valve open will draw the level in the master down about half way.
>
> George Patterson
> If a man gets into a fight 3,000 miles away from home, he *had* to have
> been looking for it.

  #26  
Old December 6th 04, 02:23 PM
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