Knock-back mechanism on brake piston

[Fishcop]

Hi All

Possibly a question best directed at Eric Shea or other brake/racing people... I threw my SWB brake pistons into my vibratory polisher for a clean up pending caliper restoration and have found that the walnut has gotten into the knock-back mechanism of the pistons. The rear 38mm are easy as the knock-back mech comes out with a circlip; but the 48mm pistons don't seem to have the same arrangement and I can't quite figure out how to remove. Anyone deleted the knock-back mech on their M calipers?

Cheers

[Fishcop]

Okay, a lot of reading on PMB's website indicates that the knock back mechs in these pistons are not removable and mine are all now full of walnut grit

[edmayo]

You just need to find out what dissolves tree bark!! Mild acid of some kind?

[Chuck Miller]

You just need to find out what dissolves tree bark!! Mild acid of some kind?

Alcohol is the solvent for natural resins.........

[Fishcop]

Alcohol is the solvent for natural resins.........

Well I've been working my way through a bottle of Ardbeg single malt... So far I haven't melted!

I'll give mentholated spirits a shot on the pistons, otherwise I might have to order a set of new ones from Eric (sans knock-back).

[edmayo]

OOOOPS! our mistake John,,,,thought you were needing solvent for the pistons

[Neunelfer]

Sorry for the delayed reply. Chucks note sent me on a college flash-back... alcohol and resins

I generally don't encourage people to "remove" the pins or knockbacks. A mild acid (wait... what? more college flash-backs) should do the trick and make sure everything is clean then, a solid spray with a brake cleaner straw nozzle and you should be back up and running. Porsche/ATE generally didn't have knockbacks in two situations:

1. Race cars (some 908 calipers, 917 calipers etc.) and,
2. Later cars with booster circuits.

That said, there are a "LOT" of guys out there with A-Calipers on the front of their early 911's (Mr. Miller). It simply requires a stab or two to make it right. The best thing for a early non-booster car and no knockback mechanisms would be a residual pressure valve in line. This would keep 2-3 PSI on the line and maintain proper pedal feel.

Back to the OP? - I'd be inclined to have at it with a high pressure air nozzle and a can of brake cleaner. If it's really packed in, try the mild acid (not the brown acid).

[Fishcop]

Thanks Eric.

Fortunately I pretty much did as you suggested and blew everything our with compressed air and brake cleaner. The brakes went back together well and appear to be functioning