[DeTomaso] Gotta love professionals!

[asajay at asajay.com]

Aha!

It turns out I -did- know what you were talking about.  I just didn't  
figure how it was affecting the parking brake, so I didn't think we  
were talking about the same thing.  I'm getting ready (once done with  
all the rebuilds) to write an article for the newsletter.

What I was failing to put together, was how it affected the parking  
brake.  Once you explained it, I was like (duh).

Thanks Mike.

Here is a quick description of the mechanism which I will document and  
put into a newsletter article later on.

The pad facing piston has a rod that is serrated at 0, 90, 180 and 270  
in the shape of a cross.  The parking brake piston has a sleeve that  
is serrated every 90 degrees at 45 degrees from the opposing piston.   
Thus, when the serrations are engaged, the pistons are essentially  
locked from closing on each other, but there is some kind of ingenious  
mechanism that allows them to "ratchet" away from each other.

The reason you have to rotate the pad facing piston 45 degrees is to  
disengage this meshing of the serrations (they actually look like  
threads) so that you can collapse the pistons together.


Now, according to the photos you sent, the slot on your pistons is a  
lot deeper than on mine.  The tab looks essentially the same as on my  
stock pads.

The pads from Porterfield contain a PIN on the backside of ONE pad.   
When you buy a set (both sides of the car), you get two pads with pins  
and two without.  In my case, I have to file the pin down enough to  
engage the slot but not too far to let it slip.  So really, it's a  
"piston anti-rotation device," and it -is- pretty important.

Oh, and so apparently, if you order the Porterfield pads on line,  
without actually talking to anyone, you -should- be getting the right  
thing, pin (tabs) and all.

:)
Asa Jay

Quoting MikeLDrew at aol.com:

>
> In a message dated 7/28/08 12 25 11, asajay at asajay.com writes:
>
>
>> What tab?  And how does it affect the parking brake function?
>>
>> Photos would help.
>>
>
> Dude!   Where you been?
>
> The piston on the rear calipers has a slot in it.   This slot engages a tab
> protruding from the backing plate on the pad.   There is some sort of magic
> going on inside the caliper, whereby the tab on the pad keeps the piston in a
> certain orientation, and it can not be compressed.   If the tab   
> isn't there, the
> piston is free to rotate as the brakes are used; if you're unlucky, it will
> rotate the requisite 45 degrees, which then disengages it from the   
> parking brake
> ratcheting system and allows the piston to be fully compressed, and the
> parking brake doesn't work properly.   Hydraulic pressure will let   
> the rear brakes
> continue to function with the pedal, but the piston will be disconnected from
> the mechanical parking brake mechanism inside the caliper.
>
> I admit I'm a bit fuzzy on the specifics; the shop manual does a good job of
> describing what you have to do when you install new pads (insert screwdriver
> into slot in piston, rotate 45 degrees, fully compress piston, then   
> rotate back
> to original position), and of overhauling the calipers, but it doesn't
> sufficiently explain *why* you have to do this.   I do know that if   
> you just try to
> compress the piston using a C-clamp, without first rotating it to unlock it,
> you *will* break the mechanism, and parts are not available.
>
> I (of course) have a spare set of rear calipers, and I really should take one
> apart to gain a better understanding of how the system works.   Someday...but
> in the meantime, RTFM (of which you have plenty!) and you'll be fine.
>
> Here's a photo of the stock, original (and worn-out) rear brake pads mounted
> on one of my calipers.   Both pads are made with the tab on the   
> backing plate;
> the non-functional outer pad is shown in this photo for clarity of the very
> slightly raised, rectangular tab directly in the center of the backing plate:
>
> http://members.aol.com/mikeldrew/RearBrakePad.jpg
>
> And here's a shot of the caliper (complete with primer overspray, a legacy of
> when my car was painted 12-13 years ago), which shows the vertically oriented
> slot in the piston that the corresponding tab on the inner pad engages:
>
> http://members.aol.com/mikeldrew/RearCaliper.jpg
>
> If your aftermarket pads are lacking this apparently important tab, several
> people on this forum have made their own by drilling a hole in the backing
> plate, installing a screw, then filing the top and bottom edge of   
> the screw head
> so that it forms the shape of a square-ish tab.
>
> Mike
>
>
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