Piston Slap: Poppin’ caps on a disc brake conversion?

John writes:

I put disc brakes on the front of my ’56 Nomad a while back, and the passenger side grease cap keeps popping off. What would cause that?

Sajeev answers:

That’s a good question, but did you mean the bleeder screw’s cap on the brake caliper? Perhaps the brake swap can affect a greaseable ball joint, but I’m gonna leave that up to the Hagerty Community to answer and/or roast me in the comments.

Provided I am right, did your kit also come with Speed Bleeders or equivalent? This speedy way to bleed your brakes includes a check valve to ensure a one-way flow when bleeding brakes., but I’ve heard they can also leak even when fully closed. If so, that would cause the rubber cap at the end to blow off.

Even if you aren’t running speedy bleeders on your brake calipers, there’s a chance that heat cycling will make it easier for a cap to come loose, especially if either surface has residual fluid on it. (Or grease from your hands, you never know!) I recommend cleaning the bleeders with brake cleaner, letting them dry, cleaning them again, and soaking the caps in brake cleaner before wiping them down. And maybe check the torque on the bleeder, you don’t need much torque but you might be lacking just a little bit!

With clean surfaces in play, I bet your Nomad’s bleeder caps won’t pop off any more. If you have speed bleeding caps, maybe consider replacing them with conventional bleeders, too. If all else fails, let the caps pop off, and just buy extra bleeders when needing to do a brake work, they are cheap and easy to replace if needed.

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