What could cause me to lose power steering under heavy braking?

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Its hard to diagnose something like that without knowing a few things. how hard are we talking. Hard braking from high speed, harsh braking, locking up the wheels braking? Has it always been like this, or did it just start doing it? The power steering runs stictly off of a belt driven pump and totally separate from the braking system. Now mechanically they are connected at the spindle/hub/rotor assembly in which case, it could just be the laws of physics or friction to be more precise. The harder the braking, the harder the steering.
 
Maybe 60-80% braking (slightly aggressive braking coming up to stops signs and such). And then my steering gets very hard to turn. (kinda like a non-powersteering vehicle when sitting still). I've only had the car for a few months now and it has always done it that i know of. I didnt know if it could be something to do with vacuum or what. Like i said, theres about two inches of play in the steering. I'm not sure if that is in my steering pump, rack, or tie rod ends. My steering fluid level is ok. I'm gonna double check my brake fluid. I have an excellent 90 gt parts car that had tight steering before being rearended. I want to just replace everything from the steering shaft down and see if that helps. Any other ideas or tips on swapping racks or tie rods?
p.s. I read somewhere on this site that an oil breather cap(i have a k&n) causes a vacuum leak. Could that be the culprit in any way? Thank you for your help.

Ok this idea just came to mind too. Reckon my steering rack or tie rod ends are just that bad and the extra weight on them during aggressive braking is causing a heavy feeling in the steering wheel?
 
It could be the ps pump is going bad or is bad. Usually they get noisy though. At low rpms and during braking where rpms are drastically declined the ps pump may not be sending the fluid through the system. This could give the sense that the hard braking is causing the steering problem. The ps system doesn't run off vacuum.
 
As for the 2" of play...
Is the play there with the car running and with the car off? I would check for excessive play down at the rack. Check the steering knuckle and shaft. Check where the steering link attaches to the rack. Check the rack itself to make sure it is tight. Check the rack mounting bushings. Check for abnormal wear on the tires. Grap a wheel and pull & twist it to check for play with the inner/outer tierods & ball joints.
 
There is a really lot of things that contribute to bad steering, if the car has alot of miles you may just want to replace everything at once, to keep from having to get $80 alignments over and over again.
Consider getting:
Ball joints, outer tie rods, new or rebuilt rack (which usually comes with inner tie rods), new PS pump, and any hoses that look worn.
Keep in mind, if you only get a new pump or rack (one or the other) you must make sure 100% of the fluid is out of the part that you didn't replace, because the old bad fluid will contaminate the new new parts.

I know it gets expensive, so if you have to, save up or compile parts until you are ready.
None of it is all that labor intensive, so you should be able to handle it yourself.