Front wheel pulling to one side when braking and eventually works loose?

[I like Skol]

I've not had that trouble myself - and I figure a little trouble undoing the QR preferable to it coming out when I least want it to. For this last issue I run marathon pluses...

I set my bearing cones up so they allow for the 'bastard tight' QR. The wheel shows some slight play until the QR is BT!

 

[andrew_s]

I ride a Trek X-Caliber 7 MTB ... the wheel pulls to the left whenever I brake. It eventually works loose

It's because you've a front disc brake and a QR axle.

When you brake, there's a powerful force trying to push the disc side of the axle downwards out of the dropouts, and if the QR isn't up to the job, it will slip until it's caught by the lawyer lips. Then you stop braking, and your weight pushes the axle back to the top of the dropout slot.
In due course the back and forth movement loosens the QR.
If you ignore the wheel movement for long enough, the wheel will come out of the forks, you'll land on your face (probably), and the forks will get bent (I've seen it happen).

To avoid it, use a QR skewer with a properly strong grip, and put the adjuster nut on the opposite side to the brake disc.
A properly strong skewer means either an allen key skewer, or a Shimano-style internal cam skewer, done up tightly enough that it's fairly difficult to undo.

This problem is the unacknowledged reason why modern MTBs all use through axles, and those that don't have forward facing rather than downward facing dropouts. There was someone who ended up in a wheelchair because of such a wheel ejection. I believe the payout was quite a lot, but included a gag clause.

 

[Ajax Bay]

https://sheldonbrown.com/skewers.html
If you're running hubs for a disc brake rotor and QR then a sensible cyclist would not use an 'Exposed Cam' QR skewer.
Read the Sheldon Brown link through for more, and the reason.

https://web.archive.org/web/2012100...hi/julesandjames/home/disk_and_quick_release/