[200q20v] Rear Caliper sticking?

[Kneale Brownson]

I got one of the sticking rear calipers off my 200q20v that I replaced a 
couple of weeks ago stripped down this afternoon to see what's what in 
there.  The emergency brake lever turns a rod that has an offset hole 
machined into it.  A small "peanut" about an inch long and maybe a 
quarter-inch in thickness with equal bullnose ends rides in the machined 
hole and pushes on the threaded rod onto which the piston is screwed.

Because the hole is machined offset, when the lever  moves with the cable, 
the peanut rises up a little bit and that pushes the piston against the 
rotor.  There's a spring, located by an internal circlip, holding the 
threaded rod into the cylinder, and the idea is that the peanut pushes the 
rod out, compressing the spring on the threaded rod and holding the piston 
against the rotor as long as the cable is pulled.

When the cable is released, the external spring moves the arm back, 
releasing the pressure on the peanut, and the spring loading on the rod is 
supposed to push the rod back into the cavity slightly, thereby pulling the 
piston away from the rotor.

The cavity in which the peanut sits was filled with a grease that I presume 
was high-temp brake grease.  It looked like really thick Vaseline.  That 
cavity is sealed off from the rest of the brake cylinder by an 
O-ring.  Brake fluid does not flush through the cavity, and changing the 
brake fluid would have no effect upon the peanut or the rod from the cable 
lever.  As long as the external cable arm is free to turn back and forth in 
its full range of movement, the peanut should make its proper movements.

I think what happens when the brakes drag even though the cable arm moves 
appropriately is that there is enough corrosion in the cylinder that it 
creates enough drag to overpower the spring.  If the cable arm movement is 
sluggish, that may contribute to poor release performance,  but if the 
external spring can move the arm back correctly, the problem has to be a 
drag in the cylinder itself.  My poorly releasing caliper's piston had a 
few rust spots and the cylinder bore had a bit of corrosion, and I'm sure 
that's what caused my problems.

I also this afternoon tore down a caliper off my '86 5Ktq parts car,  on 
which the E-brake cable had been snipped off by a PO.   The cable arm on 
that caliper wouldn't budge.  I found that caliper's peanut cavity to be 
full of dry rust and crud, no grease to speak of.

My conclusion is that if you have a caliper that's sticking, it needs 
rebuilding.  No amount of loosening the cable arm with penetrants will 
relieve the sticking.

And the real solution to these problems is to make certain no corrosion can 
occur inside the cylinder.  That means regular (annual as a minimum) 
flushing of the brake fluid to keep moisture out of the cylinder, and 
regular inspection of the boot on the piston to make sure it's intact.





At 01:45 AM 10/18/2000 +0000, -J J- wrote:

>Alright,
>Sunday I pulled the left rear wheel off and checked out the caliper. It 
>seemd to work okay, and the e-brake returned to full off with no problem. 
>I lubed the e-brake cable lever and put the wheel back on. Monday, 
>everything was good. Then tonight i went out and on the way home i heard a 
>shudder in the rear end. I got home, and the left rear rotor was red hot.. 
>Hmm.. I know it isnt the prking brake, I set it to off when I worked on it 
>sunday, and I have not used it. Should i pull the caliper apart and check 
>the pins like it says on chris miller's page? ( I will probably do this 
>anyway) Or should i order a rebuilt caliper?
>
>      tia,
>           John Gronberg
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