don't think my tranny is going into third gear, at least i can't feel the shift. now that the excitement of owning it has kinda slowed down a bit i don't think it ever did. right now because of the headers i put on, i can't put the shifter all the way down to the lower gears and do it manually.

just a question about the vacuum modulator. can it be the cause of it not shifting into the third gear? had a problem with my 81 caddy with vacuum and it shifting into the last gear so i am wondering if i have a simmiliar situation.

little clearer here. 81 caddy was not getting enough vacuum to it. on the 75 i am wondering if the modulator is bad or is it a part that either works or it don't? can it work up to a point and then foul up may be a better way to put it? thanks

Does the car have a good, solid reverse?
Does it shift 1-2 on time?

Reverse and third use the same clutch-pack, so normally, if there is a problem with the clutch-pack(clutches burnt, seals bad, etc.), you'll have a problem in both gears.

If the 1-2 shift is on time, but you have no third, it could be a stuck valve in the valve body. BTW...doesn't your '75 have a 400 in it? It should.

It is possible the modulator is bad, but a 1-2 shift that is on time would tend to confirm the mod. is working. But...anything is possible. Pull the vac. line off of it, and see if any fluid runs out. If it does, it's bad.
You may also have a vac. leak, either to the mod., or somewhere else in the car, that would limit the amount of vac. available. I've seen the metal line from the engine to the mod. rust out, and have small pin-holes in it. There was also a post recently that had a similar problem, and it turned out that the modulator itself had a hole rusted in it...in the metal "can" itself!
Make sure you have good vac. at the mod at idle, and at a higher rpm(1000-1200rpm). A vac. gauge is your friend, here.

Since a '75 is supposed to have a 400, another possibility is the kick-down, or passing gear solenoid inside the trans. It could be malfunctioning, stopped up, or leaking, and cause a no 2-3 shift. It could also be that the switch that operates the solenoid(on the accelerator pedal) is out of adjustment, or even bad.

hth