Rear Brake Caliper Questions Options

[ljubodraz]

Firstly, many thanks to Eric Shea and the rest of you who have written threads that are helping me rebuild my rear brake calipers on my own.

I've got a few questions so far as I wait for parts:

1. What is the purpose of the mechanism within the brake pistons? Does it allow the piston to extend and retract beyond the limits of the adjustment bolt that threads into it?

2. Is there anything I should do to test that it's working properly?



3. I see that Eric Shea recommends and uses Permatex Ceramic Extreme Brake Parts Lubricant for assembly purposes. I've also seen other threads saying to only use some hard to find $$$ ATE paste. Aside from lubricating the parking brake mechanism that pushes the internal bolt, can't I (should I) simply use brake fluid to lube the pistons and seals during assembly? Unlike Eric's calipers, mine won't be sitting around too long before being installed preventing the brake fluid from absorbing water.

[Jeff Hail]

All calipers in good operating condition self adjust for wear...sort of. Pistons just keep extending. Single piston, dual and mega-binders, fronts, rears. As the pad wears the piston moves outward in its bore. Pad sits up against the rotor.

Parking brake mechanism:
Parking brakes incorporated into the caliper operate a little differently. They use a lever, cam, worm gear (different designs) etc to mechanically squeeze one piston without hydraulic assistance.(one side only)

Both pistons have a manual adjustment. The threaded adjuster has limits to its adjustment. As the pads wear or if both sides are not adjusted properly the cups can run off the threaded adjuster in an extreme case. Visualize one piston extended 3/4 of the way out of its bore and the opposing piston only 1/4 way extended? Pads will wear with a cant and the friction material end up a wedge instead of a puck, rotors cut by the friction material angle of wear.

Vent:
Setting clearance is to achieve a little slack between the two sides of the caliper. From a safety perspective it helps the piston so it does not seize in the bore since it keeps moving. In theory the aim is to have the rotor perfectly centered between both pads but in real life with the addition of the mechanical parking brake it gets help. The rotor is "nudged" lightly against the outside pad.

The answer to your question is yes they do self adjust... to a point and then the answer goes to sometimes and then no. This depends on how they are initially set up, condition and kept in adjustment.

On the knock back side you are back wards. The rotor under deflection pushes the piston back in its bore. The piston may not return quick enough so the little KB helpers (springs/washers) provide a dampener for the pad "keeping the pedal". Floating calipers do not tend to have this problem because the caliper follows the rotor. Fixed caliper bodies its the pistons that float back in forth and this is where problems begin.

At the end of the day the knock back mechanism is passive. The vent clearance is not constant throughout the service life of the pads so adjustments are needed at prescribed intervals. ATE designed a good economical caliper with a tiny amount of drag built into pad to rotor contact and Porsche took it back by the vent clearance. The published clearance changed over time which means something for good reason moved Porsches cheese.