Hi TimberXX
I have no experience with the BCS 38'' lawn mower myself, but I have used the quite similar Zanon ZCR 800 lawn mower for more than 225 hours over the last 7 years now, so I hope that some of my experiences might be useful to you.
Mowing very tall grass and hardwood brush as you mention in your first post, is pushing a lawn mower beyond its limits I think. As you can see in my thread on the Zanon ZCR lawn mower, I was struggling mowing "just" 1 foot tall grass on my lawn, after a vacation a few years ago. I fully agree with cmyoung2 and jbradley, and I'm not surprised to hear that you bogged down. Did you actually mow your pasture with the lawn mower, or did you mow your lawn with it?
The way you describe your pasture, I would consider mowing it with either a sickle-bar mower or a drum/disc mower if you want to remove the clippings, or a flail mower if you prefer to leave the clippings on site. With the amount of clippings that you will probably have on your pasture, I would be reluctant to leave them in place, as the wads left by a flail mower - or especially a lawn mower, will take a long time to decompose and disappear.
When I mow my lawn with my Zanon, I usually run my BCS 740 in third gear with around 3/4 throttle, which gives a speed that suits an old man like me well. I hardly ever use the steering brakes when mowing my lawn, as it only takes a slight sideways pressure on the handlebars, to steer the tractor in the desired direction. It is important though, that the caster wheel spindles in the front are well greased, allowing them to swivel freely.
If you use the mower for what is was designed for, that is mowing the lawn, I hope you will be pleased with it. For mowing your pasture, it might not be the best choice.
Best regards
Jens