Brake Pedal Clunk

Two things, when it clunks, first, does the pedal drop just a bit, but still feels firm and two, only happen when it's running, i.e. when the booster is charged?

I have a similar issue in another car ('74 Alfa 105). I rebuilt the calipers and replaced the master. Waiting on a new booster. I'll let you know.

I have only focused on the problem when the car running and the booster is charged with vacuum.

When the problem happens, the pedal clunks immediately and then works normally.

I ran the problem by Wilkinson this morning and we are thinking it could be the master cylinder spring not returning fast enough.  If so, it might be letting the booster's push rod (looks like an engine valve) drop out of the socket in the master cylinder.  Then when pedal is pressed, it finds the depression in the master cylinder socket (clunking into place) and works normally.

I have another new master cylinder (actually another rebuilt unit) and plan to swap it in and see if that fixes it.

I wondered about the same potential problem, the push rod not seating into it's receiver every time. Upon removal of the master i could see no evidence of such slippage. Checked under the dash to see if anything funny might be happening at the connection to the pedal. All appears in order.

I wondered next about an internal seal leak within the master, so that got renewed, but alas, no joy. I did notice though, that I could not reproduce the symptom unless the booster was charged, so that's next. In fact it arrived yesterday.

I'll know by the weekend.

A difficult problem to diagnose by long distance. I've seen such things caused by- for example- a loose front antisway bar that moves around when the car initially dips its nose on braking. I've also seen worn lower (rubber) shock bushings cause a clunk. It may not be tied to the brake pedal itself but to chassis motion up front. Look for signs of wear in things close to each other that shouldn't ever be touching.

It looks like I was able to isolate the problem and fix it.  I could duplicate the clunk by pumping the brake pedal with the car running and in neutral.  I got a helper to sit in the car and pump the brakes in the same scenario, and the helper could feel the clunk as well.  I next checked around the car while the helper pumped the brakes and I could feel the clunk when I put my hand on the booster and concluded that the adapter shaft and/or plunger shaft might be binding.  Upon disassembly of the adapter I found it to be somewhat dry and resistive to sliding, and the flat part of the plunger shaft somewhat dry and gunky.  I cleaned everything (again) and this time used engine assembly lube throughout the setup.  On the bench the adapter shaft began sliding smoothly unlike before.  Everything is back in the car and the clunk is gone.