Thanks for the info. There is a number on the outside of the cylinder, but it looks like a serial number or something. However in taking it apart again, I noticed that one of the rubber cups is marked 1 1/4. Also I managed to get a micrometer inside far enough to get what I think is an accurate reading. It also said 1 1/4. I just ordered a repair kit from Jim Carter. Using a light on the outside and a very fine wire, it looks to me as if the front rubber cup is not clearing the very small hole that lets fluid down into the body of the cylinder. When I have the cylinder in a vice it will work, but there is no pressure on it. I think possibly on the truck there is enough back pressure to cause the cup to close the hole off.
On a different subject: In the shop manual there is a picture of a hydrovac that shows what looks like a plug to put brake fluid to lubricate it. Also the manual calls for that to be done. Someone on this site also said that was correct. However, the unit I have does not have the plug. Seem strange, but there is no plug. This is a unit that I bought when I was rebuilding the truck, and supposedly it had been rebuilt. I think it came from NAPA, but I'm not sure. I started to take it apart one time, but didn't have wrenches large enough, and didn't get that done. I did get the vacuum tank off and that had a lot of dirt in it. Leads me to think someone may have given it a coat of fresh paint and called that rebuilding. Is there another way to lubricate then inside of the unit? Thanks Allen
