Transmission help I have just put a transmission in my 2000 gmc truck. There is one vent line on the top of the transmission that I am not sure where it go. I think It goes on the back of the engine but Can not see any where that it could go. Can someone please help. Thank You

no such thing , vents yes, ATF line yes , vacuum no.
wires yes.
only super old transmissions use vacuum modulators
let me guess you see to hoses an assume vacuum?
the metal in pair is tranny cooles
the service manual covers this.
2003 GMC Envoy 4,2 2wd'
gas 4L60E TRANSMisson shifts by TCM computer, totally.
no vacuum modulator there ever.
learn to follow line the other direction to the cooler (2 kinds)

your sure its not the tube that goes to very top center for vent tube ,if it isnt that one it could be for the vaccum port on side of transfer case for 4 wheel drive vaccun switch usually most of newer vehicle have went to electronic soleniod

first check the transmission oil level with the dipstick. make sure the truck is on a level surface. there is level lines on the stick to show low and full levels. the brand of oil will not cause this but make sure the oil type is right. it should be a Dexron type oil which is for GM vehicles. if the vent tube for the transmission is plugged it can cause the oil to purge. it could be a hole in the oil pan or the pan gasket is damaged leaking the oil out or even a trans seal is bad.

They are controlled by vacuum ... a line must have been disconnected or broken when the trans was replaced. Remove the dog house and look for a small vacuum line broken or pulled apart.

Common for the transmission line to the radiator has failed due to corrosion. You can replace the line or section it off bad part with a new line, just make sure you use compression clamps. I had the same problem and i ended up running a new line with a inline remote transmission oil filter. B&M makes a universal remote transmission oil filter and runs around $45.00. Good luck and hope it's just the line that went bad, keep me posted.

If the transmission is over full this will happen, if the transmission overheats this will happen, check the level carefully by the instructions on the dipstick to ensure that it is filled properly. There is also a vent on the top of the transmission, a favorite home place for dirt wasps, make sure it is clean. If you think this problem is over heating, check the oil cooler for obstructions pinched lines etc., I put an external oil cooler on my transmission to provide more cooling capacity, though it is not truely neccessary. Hope this helps, let me know.

There are two ways to changing the transmission oil. If your going to change the transmission oil and filter, your going to have to drop the transmission oil pan to get to the filter and the oil will drain from there, but keep in mind that it will get messy. The other way to replace the transmission oil if that is all your going to do is to get a large bucket and a small hose the size of the transmission oil line. You can disconnect the transmission oil line from the radiator on the passenger side and connect the hose to the line and the other end to the bucket. Start the truck and try to do this with two people, one to hold the hose and one to start the turn off the engine when the transmission is almost out of fluid. DO NOT RUN THE TRANSMISSION DRY, so make sure the other person understands you when you tell them to shut off the engine. This will ensure that you replace most of the fluid in the transmission, once most of the oil fluid has been drained from the transmission, just reconnect the oil line and refill the transmission fluid through the oil dip stick. Good luck and hope this helps. 
Automatic Transmission, 4L60-E Initial Fill..........4.7 liters
Automatic Transmission, Total Fill 4 speed 4L60-E..........10.6 liters

I believe the line you are talking about is a vent line not a vacuum line. It should go tp a metal tube that is in a "U" shape. This bolts to one of the bolts on the transmission.

Does it sound like a loud winding sound coming from the front right side? I have a 2001 Jimmy and the 4wd tries to engage when driving up steep hills and going at highway speeds. The actuator vacuum switch engaged when the transfer case vacuum switch sends the vacuumed air to the actuator. What is happening is their is either a leak in the vacuum lines, a faulty actuator switch, or a faulty transfer case vacuum switch. When the jimmy has enough air pulling in, the extra vacuum is trying to push the actuator switch in when it is not supposed to. It is a much cheaper fix than a transmission. GMC should have recalled the whole 4wd system on any of the S10's, the JImmy's, and Blazers.

I'm told and read a sun shell may be broke. Common problem for this vehicle. Shop will possibly rebuild and use aftermarket kit parts to correct problems the transmission had originally. Cost is probably between $1300 to 1600 to repair. Good luck I got the same problem on a 97 Jimmy and 3 shops said the same thing.