Erik, I checked and honestly it musta been changed or Im just missing seeing it. Either way 1/16" wont matter Id think, it matters when you step up 7/8" to the 1" of course. As for measuring it, Ill be doing that shortly just to make sure.

Checked the booster rod inside, still has the factory paint on it. What has been happening is pedal travel directly correlates to outside temps (after I adjusted the booster pushrod). Was fine one day cause it was cold-high like 40F at the most, warmed up to about 55-60F one day and it was over adjusted. Infact I have unadjusted it several times now since it got warmer.

So my best guess is either where the pushrod is-that rubbers bad and leaking (doesnt seem like the boosters leaking at all). Or inside the master the rubber is expanding and contracting rubber seals. Either way masters like 100 bucks vs 250 for a booster. I cant find a t100 in a junkyard here either, so its new or bust it would seem for both. Theres literally nothing else old on the brake system otherwise.

A 1-1/16" piston has about 13% greater surface area than a 1" piston, so your pedal firmness/height will increase by that much, as well as how much effort you have to use press the pedal to stop the truck. It will definitely be noticeable, so if that's what you're after then you'll probably be satisfied with the change.

Thanks for the info, I will take that into account.

Looked all over the master, there are no size markings. Just the following: What appears to be 6E10 NS4 on the front and on the bottom it says Aisin.

I tried to measure it, theres a snap ring in the way, measuring outside of it I get roughly 1.17 inches which seems way to large. Measuring inside the snap ring gets me 1.063 which is 1 1/16". So it would appear I have the larger master I think. I mean unless I take it all apart it seems I wont know for sure. Im just gonna buy the smaller one, I suppose I can always exchange it.

Going to call toyota as well and see what they can tell me.
Edit: Call yota, they can only tell me truck is spec'd for 1/2 ton master. IDK..

Also: The master shows no outward signs of leaking at all, cant tell if its leaking internally.

The booster, when you grab the pushrod air escapes-normal?