Adjusting the brake booster rod

[LoveTheGMNUMMICars]

Hi!

My 2010 2.4L Vibe had a leak probably at the gaskets of the original main cylinder's reservoir (fluid leaked onto the brake lines and on the side of the master cylinder), so I'm just replacing it all.

With a change of master cylinder, you have to check the depth of the brake booster rod--using a metal gauge included with the new cylinder--and adjust as necessary.
The rod has a 7mm head which you adjust, but you have to hold onto the brake booster rod (or it just turns).

I have tried various things, including this from a public website:

One way of trying to hold the brake booster rod!

Screenshot_20250604-110814_Firefox.jpg (643.68 KiB) Viewed 34 times

The problem is that the serrated portion is deeper than in this photo and the hole is ≈2‐1/4" in diameter, so getting even a boxed-end wrench, etc., in there and not blocking the tip doesn't work. I was able to recreate a set-up like the photo, but the rod kept slipping.

So I ordered the tool--a discontinued Kent-Moore tool, J-37767 - Brake Booster Piston Rod Adjuster, which looks like this

KM J-37767

Kent-Moore-Tool-Div_J-37767_WRENCH-2243020222.jpg (17.58 KiB) Viewed 34 times

and of course is meant to hold that rod.

But have you ever successfully adjusted the brake booster's piston rod withOUT that special tool?

I'd love to know how!