This thread is so old, it is starting to stress me out!
"Pulling hard under braking" can't really be caused by the rear BRAKES. The car trying to SPIN because of a locked rear is certainly possible - but rear brakes don't steer a car very effectively, unless you count skidding/spinning as "steering".
Even if you have only one FRONT brake working at all, the wheel will try to yank out of your hand - but you should be able to steer through that. If the wheel is straight, the car should go straight, despite that it is tugging at your hands.
If the brakes are hot, they are working. If one is way hotter than the others, it is working harder and should be considered suspicious.
If you can't steer through the "pull" with a straight wheel, it means one of your wheels is steering because of compliance ("flex" somewhere). Front bushings and subframe would be the first place I would look - this assumes all of us Pro Commuter Car Cup drivers would already have found something as obvious as a sloppy tie rod end or balljoint.
Like Wheeler said, a cracked front subframe will PULL - the harder you push the brakes, the more you'll have to countersteer - that is because the front wheel is being "peeled back" and steering on its own due to the braking force. The good news if you keep it up, it will eventually completely fail. I know because I had the pleasure of driving an NB that was fine, then started to tear, and then completely disconnected, all in one enduro. The ones I've seen fail where the "pocket" attaches to the lower subframe, by cracking right around the weld. The ones I see failed are the front pocket location.
Your front toe only matters in terms of total toe because you as the driver are "centering" the road wheels as necessary to go straight. The two sides being different only matters as regards how straight your steering wheel would be. So, 5 mm total toe out in front isn't ideal, but it isn't a huge deal. It won't make the car undriveable or even very weird. 90% of drivers would never be able to tell.
The rear has 6-ish mm total toe-in, but thrusted "rightward" - meaning the car will dogtrack a bit, but again, this isn't a huge amount. It would cause the car to "hook" a little left under braking and into LHers, and "push" a little into RHers. 90% of drivers wouldn't be able to tell, and any of 6 other things could counterbalance this effect.
HOWEVER: How are you measuring the X mm? Front tape reading minus back tape reading using the strings at the wheel lip? X mm at the wheel isn't X mm at the end of a toe plate. This is why "angle" is really a better measurement, since 1/16" of "toe" on an old Mini Cooper with 8" wheels is 3 times more ANGLE than 1/16" of toe measured with 2 foot long toe plates!