308 Rear Caliper Adjusters have stopped working

I've just been and put new pads in the rear of my 79 308. The new pads only just fit, so the pistons needed to be fully retracted to make room. The calipers were rebuilt a few months ago, so there were no dramas moving the pistons.

What has now happened, is that none of the 4 adjusters seem to do anything. Its almost like they have become disconnected from the pistons, which doesn't make sense, as the adjusters should be as deep in the pistons as they go. I'm trying to recall how the threaded part of the pistons looked - I've got a feeling that the thread is an insert in the piston, and not directly threaded into the piston itself. In this case, is it possible that the insert has been pulled out of the piston, maybe when he piston was fully retracted?

Any other ideas what could cause this? I'm guessing the calipers are on their way apart again....

Andrew

 

I finally got this sorted, and a few lessons in here.

The adjusters have a threaded shaft that can drive the piston in and out. Inside the piston is what appears to be a spring steel plate (just visible at the bottom of the piston) with an attached threaded boss, which is what the adjuster threads into. This plate has a number of ears which contact the inside wall of the piston, and is a friction fit inside the piston. This presumably allows for some lateral hydraulic movement even when the adjuster is locked.

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If you retract the piston too far, it is possible to bottom the inside flange of the adjustment screw against the threaded boss. What happens then is that the whole mechanism can turn inside the piston, which is why the adjuster stops moving the piston. This is not a good thing, but in most cases seems to free up once there is some clearance (ie. remove the pads, and maybe apply a bit of hydraulic pressure). Worse is if the boss locks onto this flange, which is what happened to one of mine. Luckily this was one of the outer pistons, so I had to remove the piston complete with the adjuster to free it. This is still not so easy, as there is nowhere to grip the internal mechanism. I ended up pulling out the tang of the internal spring, using a tiny pin punch in the tang hole to lock the internals, and an impact driver on the adjuster. I suspect now an impact driver might be enough, even with the piston still in the caliper.

It's definitely much easier not to do this in the first place!

In my case, I had just rebuilt the calipers with a kit from Superformance. I don't know know similar this is to other kits, but with the pistons fully retracted, the dust seal extends just past the contact face of the piston. With fat pads, which need the pistons fully retracted, this means there is a high risk of damaging the dust seals when inserting the pads. In fact I had damaged all 4 dust seals, and needed to pull the calipers off again and replace these. Interestingly, the new seals appeared to sit flatter when retracted, and are not so exposed.

The pads I was using were standard DB2 pads from my local supplier, potentially spec'd for the older 911's. I don't know how these compare to the thickness of other pads, but I decided to not risk both problems again, and shaved 1mm off the face of the pads before reinstalling (this took the friction material back from 9.5mm to 8.5). This allowed the pistons to be extended sufficiently that the dust seals were totally safe, and also well away from the risk of losing adjustment. I'm just finishing bleeding the brakes now, and everything is looking good.

Andrew