1991 300E

First I want to ask only for reply of people who have really flushed their cooler!

1)I'm in the middle of replacing my transmission. Reverse is gone at 216,000 miles. I've been told typical problem on the car and the miles were right for reverse failure.
2)I've disconnected the lines to the tranny cooler (which ofcourse is part of the radiator)
3)I've used the typical transmission cooler flush agent you can find at most part shops.

Now to finish it, I'm using a hand pump to flush again with transmission fluid, Now my question!!! Those of you who have done this..... Did it take a serious amount of pressure to get the fluid through it??? I don't know what the inside of the cooler part looks like, but its not easy getting the trans fluid through. It is taking more force than I'm used to, but I'm not a Mercedes guy, so I have no idea what to expect. It will flow with quite a bit of force, but I'm wondering if its normal, or time for a radiator replacement. No need to spend money, if I don't need to.

Anyone done the same process?????????