1993 CARAVAN

I have 1993 Caravan that has over 240,000 miles on it. The ebs unit has been worked on twice since we got it in 1995. The problem is that I lose my brakes at any time and the ebs brake light's come on and so does the parking light brake. By losing the brakes I mean I can push down all the way on my brakes and at the end of the push I can get alittle brake to slow down but it is like being on a slow coast but I can get it stop. Then I can drive like this for five miles and the ebs light goes out and I have brakes. It has progressed to the point that I never know when it is going to happen and can't chance driving it. Took it to a dealership and they worked on it and said there is a filter in the braking system that is so old that they can not get a new one. They said they cleaned it and tried to save and fix the filter. I picked up the van and within two day's it was up to the same old thing. Now they say that the fluid within braking system is so dirty that it is clogging up the system. And they want 400.00 to drain the system. Does this sound like it could work? Or is there really a filter in this brake system. Need some help and input. Really hate to sink alot of money in this car but I keep it because the engine is still good. Ty for any input.