What would be your next step for dealing with this brake booster/master cylinder issue?

Sounds like you've diagnosed the problem if the washers relieved the symptoms. The shape of how the acorn nut mates to the cylinder may matter. Did you retrieve the original booster/ push rod and compare? It sounds like there may be dimensional differences in the new old booster/parts.

 

Any progress or is this a project for the weekend?

 

Update: I drove 7 hrs home on Xmas day by shimming the MC off the break booster with 2.5 mm thick washers. Since that wasn’t a permanent solution, I’ve continued troubleshooting over the holidays, from my family’s house while working remotely.

1. I filed the acorn nut down, reducing the length by approximately 2.75 mm, more than the thickness of the washers. The breaks were seizing again.
2. I thought the issues must be the master cylinder so I bought a new Duralast MC from Autozone for $180 with tax. Popped it in, a friend helped me bleed it. Still seizing.
3. I recovered my old brake booster from Autozone (long story). Without reinstalling it, I used the gauge (linked above) to measure the depth of the push rod. It’s approximately 5mm closer to the MC than my new booster’s filed down push rod is. If anything, I would expect I should adjust the push rod to make it longer, not shorter.

I am guessing that when I tested the brakes with the washers, shimming the MC from the booster, I didn’t heat them up enough to make them seize.

The only options I can think of are:

A) the push rod is still too long and somehow the depth on the old Booster changed when I uninstalled it. I can’t make the push rod any shorter because by now there is almost no more acorn nut to file down.

B) The push rod on the booster is defective and sticking.


Any more ideas?

 

Was there possibly a spacer between the old booster and the firewall that was left put? That would decrease the distance between the brake pedal arm and the inlet of the master cylinder making it tight in there..

One time I replced a vacuum booster on a old Lexus GS430, neglected to notice the thin spacer behind the booster, and the brakes never felt right until I figured it out and put the spacer back in.

If shimming the master away from the booster seemed to help, try shimming the booster away from the firewall.

Or maybe try shortening the pushrod at the brake pedal arm under the dash.. but sometimes that can lead to more trouble.. I guess it would be easier to mess with that pushrod than pull the booster out again to shim behind it.. good luck

 

Is the nut on the end of the pushrod too wide for the ID of the MC and is interfering so getting stuck? Is it possible the issue of the brakes seizing is separate from the booster and MC issue? What brakes are seizing? The fronts or rears? Was it not seizing before the booster was replaced With the old booster it didnt seize and you just had a pedal that was hard to make brake the car?

 

I moved the spacer from the old booster to the new one before installing it. It seems like the push rod is fixed in place, irrespective of the distance between the booster and the pedal. I adjusted the push rod length on the pedal side and I don’t think it affected push rod position.

I measured the nut on the push rod and it easily fits inside the MC. The front brakes are definitely seizing, the back brakes may be seizing too, but to a lesser extent. This issue was definitely not happening prior to replacing the booster. It’s possible the issue is unrelated to the booster/MC. I’m going to try replacing the flexible brake lines going into the front calipers next.

 

It's the booster. It's either for a different vehicle or not adjusted properly. There was no problem until that component was changed

 

As for being faulty, it’s possible that after I compress it, it catches and fails to rebound, maintaining pressure on the brake pad. But
what I don’t get is why would this only happen when I intentionally ride the brakes and heat them up? Maybe all that working the brakes gives it more opportunities to catch?

As for the adjustment, there’s only one thing that can be done, which is adjust the push rod to be longer. This definitely won’t help.

When I adjust the brake pedal, it doesn’t seem to affect the push rod’s position. Could I be missing something with the brake pedal adjustment?

 

That's why I've been asking if you maybe just have a separate caliper problem but you indicate problem didn't exist before. BTW purposely heating brakes could cause unintended rotor problems. It sure does sound like the booster is somehow not right. Like wrong booster (longer dimension) or maybe vacuum locking on but never seen that before.