T68, my 68 ACCO needs braking too, my brake experts says ya can't tell by pulling the plunger out, you must take master cylinder out and in my case give it to him, as there are 3 sizes I forget the numbers but each is only a poofteenth different. My mane has change over and parts as required. I just gotta get unbusy.
The bigger trucks are all 1 1/2",dont recall the 1300 size .Wasnt hard to get tho ....What normally happens with master cylinders is they get a ring of rust round the outer part of the bore where the wiper rubber sits ,and causes them to drip fluid on the cab floor.
In the past Ive relined master cylinders with bronze bushes from Bearing Service.....the thin stainless tube the reconditioners use is unobtainable outside the trade,for some reason......but I admit I havent tried the net lately.