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Momentary brake pull

Brakes

Vehicle Information: Model: 912; Year: 1967; Body Type: Coupe; Engine (type, size, modifications): 1600; Total Mileage: lots;

I have a '67 912. When I apply the brakes suddenly the car pulls to the left for an instant. It then slows smoothly without pull to one or the other sides. If I apply the brakes gradually I do not notice the pull, and if I press hard after applying gradually at first, it does not pull to the left. I had the calipers apart cleaner the pistons and caliper (there was no rust inside), and replaced all rubber parts. Everything has been polished with fine wet emery cloth and flushed several times. I've bled the brakes three times. The first time passing several ounces of fluid and the subsequent times 6 or so pumps of the pedal. The fluid is clean and does not have bubbles I can see. I've got the notch in the piston surfaces within a few degrees of 20, but they may not be perfect, as I did not use a gauge. The step in the piston surface on the left wheel pistons is not as distinct as on the right pistons due to rust, although all rust has now been removed, and the surfaces smoothed with a file. I'm considering swapping the pistons from the left wheel to the right and right to the left to see if that does anything. I could at least tell if it was the piston notch then. Can anything go wrong with the gadget inside the piston that the on the caliper slides into, that would cause my symptoms? What does that thing do? I do not know how old the front brake hoses are, they do not leak, I have had the car about 6 years. I had the suspension pan under the gas tank replaced several years ago because of rust below the battery so the suspension supports are solid and not rusted. It would be much more pleasant without the problem. Any ideas? Your comments would be much appreciated.

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