1965- Transmission problems

[BLH]

I have a 65 Tbird with a 390 engine. The transmission shifts fine with normal driving and if i floorboard it at 30 mph it locks up and takes off. However, if i am cruising as 50 mph or higher and floorboard the accelerator the engine revs high without the transmission kicking in for a second...and then it grabs but it accelerates like I am riding the brake also. Thoughts? the transmission is not slipping but rather not engaging in passing gear if that makes sense.

 

[74 Harley]

Maybe it's time for a transmission flush. It sounds like the valve body is not responding quickly. If a flush doesn't fix it it may be time for a rebuilt valve body. However keep in mind that the torque converter could also be showing signs of wear.

 

[BLH]

Thanks. I will give that a try. The fluid looks clean but I do not have the records on the last time it was changed. However, the transmission was rebuilt 7,300 miles ago.