Only took inifinity plus 1 trips to HD and local pro plumbing shops! You can get a full vacuum breaker repair kit on eBay for $12.95, but I'd rather make a zillion trips and even drive to Denver, but that's another story...
OK, found the washers in the specialty bolts, etc., section, not in the regular plumbing area of HD. You get three perfect neoprene washers in the package. They fit the plastic shaft that slides down into the input side of the valve. You can see the old, hard washer and the bare shaft at the top of the photo.
As it turned out, once the washer was installed, I found out that the O-ring had deteriorated (took very close inspection to notice it) and the whole valve top leaked under pressure. The O-ring size measures 3.25" in diameter, but it turns out that that size is tight around the groove in the lid. A 3.5" diameter (3/32" thickness) O-ring is perfect. I bought them for about $0.75 each at a specialty shop in Denver. You can buy them from several places on the web for $0.50 each, plus shipping.
The circular spring clamps couldn't hold the plastic pipes on the valve and toilet stems when subjected to street water pressure (near 100 psi), so I bought some small hose clamps (the slotted metal with adjusting screw kind) to really clamp them on. Worked great.
Note that the vacuum breaker valve has two arms sticking out one side--they are the toilet-side part of the seat hinge. You need to take the seat and lid off, plus two screws on the inside of the toilet, to get the vacuum breaker out. Makes sense to someone, I'm sure.
Does anyone have a source for these seat buttons? I noticed that on this toilet from a 75 Sovereign
there are 4, but in the 1977 Safari the same toilet has 6. Are we getting heavier?