Caliper rebuild question
 

I am rebuilding a set of "M" calipers from a 1966 912. After cleaning all the parts up and inserting new seals, I find that it takes quite a bit of force to push the pistons back into the cylinders. The piston slides easily in the cylinder bore, but the pin in the center of the cylinder binds on the spring in the "barrel" in the back of the piston. The binding is such that I can just barely push the piston in by hand.

Is this normal or do I need to do something to make this spring bind less on the pin?

BTW what is the purpose of the spring and pin? Is it a retraction mechanism to pull the pistons back in when the brake is released?

Bill,

Perfectly normal behavior ... that assembly inside the piston is an anti-knockback self-adjuster. It was intended to counter the tendency for cornering forces to push the pads and piston away from the rotor during vigorous driving ...

I normally use a 'C-clamp' style set of Vise-Grips to slowly push the pistons back in.

Thanks for the advice guys. I will reassemble without worrying now.

A related question. I split the caliper halves during disassembly. Any advice on how to prevent leaks during reassembly? Should I use RTV or Loktite 540 on the halves (outside of the O-rings) when reassembling?

Bill,

The proper Ate rectangular cross-section seals used to be sold at VW and Mercedes dealers, and the Porsche part for 'M' calipers is 901.351.928.10 (same as VW pn 311.615.277) ...

Just a bit of Dow Corning 4, 111, or 112 on the new seals and you won't have any leaks! Same lubricant is used on the caliper bore and caliper piston.