F250,460,C6,'88. I have been playing with this truck for several months, fixing problems. The tranny would take a long time (3000-3500) rpms to shift 1-2. 2-3 never a problem. The lines going to the modulator were cracked and I cut off the bad and it shifted fine. Changed the fluid and filter. I am always low on fluid. No external leaks. Not going long distances or high speeds. Is it a defective modulator or something else? Thank You

 

It's going somewhere, either leaking, or getting sucked up through the modulator vacuum line. How has it been shifting since you replaced the vacuum lines?

 

I'm betting on the modulator.. One way to check it is to pull the vacuum line off at the modulator and see it has transmission fluid in it, even a little is too much...

 

Also, if 4x4, check to see if it is in the transfer case.

 

It was shifting fine. Started it up today to get gas and it shifted 1-2 @30mph. After 5-6 blocks no problem

 

Very common symptom of a bad or incorrect modulator.