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1 second after you bed the pads in.
And if it is a reeeeeeaaaaaaally heavy trailer, it should have it's own brakes.
And really heavy trailers that have their own brakes, are operated from the tow car brakes......That is, when the car brakes, it causes the trailer brakes to come on and help slow the lot down....
To answer the OP's question....50kays should be sufficient kays to bed them in and tow a trailer with the car...
I really don't see how the number of k's makes a difference considering you can do 50 k's without touching the brakes.
When you talk of trailer brakes, are you talking about override or electric brakes ?
Either or......Electric brakes operate off the car brake pedal....Over ride brakes work when the trailer hitch pushes against the back of the car....ie, when the car slows down, the hitch pushes against the tow ball and pulls the brakes on.....Yes, override trailer brakes automatically come on, when the car is going down hill, but on a flat or up hill surface, it requires the car to be stopping to operate....