I have a 59 powerglide. I am just now installing it in my rebuilt baby. It was rebuilt about 5 years ago. It will shift great when its cold. From first to second and then downshift. When the engine temp gets to about 150 it will not shift out of first. I have tried a new modulator valve and it helped, but now it will not upshift after the temp gets to be about 180. I have talked to several transmission people. Some say its the transmission some say its not due to it shifting when cold. I don't think I am getting a lot of vacuum but I am think I am getting enought to make it shift.