The governor may not be fully retracting and is letting some pressure bleed through.
As a funny FYI - a local trans shops fix for a sticky 518 governor was to put one of the snake oils in the fluid, and "rattle" the governor by tossing it into park at about 10MPH. I'm guessing that particular step wasn't included in the shift kit instructions.
All Ive ever had by stuffing any trans into park
at low speed was the ratcheting of the parking paw
till it locked into park.. I dont understand what
would be accomplished by that.. other than ripping
the park gear or the paw apart.. I might try the
snake oil if I do show 2+ psi at the port... I had
this trans gone over by a shop AFTER I went through
it.. just to be safe since I've had so much of a issue
with these.. this was the first one I did myself.. the
guy said everything looked great but he felt the regulator
spring was way high.. he guessed it would be about 200#
as a guess.. all that went into the trans was a shift
improvement kit from trans-go.. I do remember a spring change
but didnt think it was the regulator pressure spring.. so
I'm not sure since I myself didnt put it in
