I have a 1988 samurai. I had to get on the brakes hard today and I almost lost it. The whole vehicle kind of swayed back and forth under braking pressure. I was able to stear and keep it ok but something is definately not right. Is there some kind of valve that controls the pressure to each side? It kind of felt like one side was getting more pressure applied than the other.

Thanks in advance.

No - there should be a proportioning valve that limits the rear pressure, but nothing for side-to-side.

You probably need to do an inspection, check for binding calipers and slides - look at the pad thickness, it should be comparable left/right & inner/outer with no significant taper - if this is not so, then something is binding somewhere.

Also check the steering linkages. and suspension shackles, bushes & U bolts - I've had a worn bush in the front end (not on a sami) allow wheel movement under braking and that caused the vehicle to pull under heavy braking.

You might also want to bleed your brakes.  Some air bubbles might be trapped in there.