Front brake calipers keep locking up!

I have a 2006 trdsport 6speed with 140k miles. Since I've had this truck (2 years) I've replaced both calipers because they have locked up. I just replaced the second one about a month ago. And now it sounds like I'm driving on the rumble strip when driving down the road but it's only randomly that it does it, though it is progressively getting worse. I can
Feel it and can see more brake dust on the passenger side than the driver. I Haven't torn it down yet but it has to be the pistons locking up again. I got the calipers from autozone. Anyone got any idea why they keep locking up? Any help would be appreciated!

 

When is the last time the brake fluid was flushed?

 

I've never completely flushed the fluid just bled it everytime and added to the old fluid. Could the fluid be dirty?

 

Open the cap and look into the bottom of your brake fluid reservoir and see how dirty your fluid is.

 

I can't really tell, doesn't exactly look clean but not just really dirty either

 

It's not usually dirt in the fluid that is the issue, it's moisture content. Dot 3 + 4 fluid absorbs water over time, reducing it's boiling point and contributing to corrosion. Flushing through with new fluid saves some headaches.

I'd be checking for wheel bearing play on the offending wheel too.

 

On most vehicles, it's recommended to change the fluid every 30k or two years. Like landphil said, brake fluid is hydroscopic (absorbs water). However, contaminents in the fluid (dirt, debris, etc.) can hinder the pressure going through your brake lines, it can also get stuck inside the caliper preventing all of the pressure going back through the lines (when you let off the brakes) to relieve the pressure and causing the problem of them locking up.