1998 Chevy Astro Pulls to right on initial braking

When I initially apply the brakes, especially over 50 mph, the vehicle pulls hard to the right. After the initial application of the brakes, it "evens out" and pulls the same on both sides. My local mechanic who
is very experienced is perplexed because he has checked the brakes very thoroughly and cannot find anything wrong.
2 years ago, I pulled the fuse on the abs system, thinking that might make a difference, but to no avail. There was no change whatsoever.
My dealer mentioned a rather expensive fix but did not want to guarantee it to work. He said there is a type of manifold or valve that the brake system distributes pressure from. It was quoted as $1500 repair. What do you think?
Thanks,
Mark Urban