Bicycle Mechanics - brake adjustment need serious help!

like raliegh71, i have a bike who's brakes will not adjust. however mine is a brand new trek fx 7.5. i have had it into the shop 3x, and i stop oh, every minute or so of riding to make adjustments but no dice. the rear brake will not release on one side, resulting in near constant rubbing. the tricky thing is, if i get off the bike & spin the wheel, it's fine. when i get on and start riding, brakes are rubbing. Help!!! i love this bike but it's becoming so frustrating i don't ride it anymore. i have adjusted it so the pads are farther from the rims, i've adjusted the tension screws back & forth about 100 times on each side, i can't get anywhere, and the mechanics at my local shop adjust it, it's great until i ride away, by the time i get home it's rub rub rub. anyone? please? (first time poster, really hoping for some suggestions! thanks all.)

http://www.parktool.com/repair/readhowto.asp?id=21

these are all the adjustments i have been doing. when i get off the bike & look at it, everything is aligned (and not just by my eye, others who know more about bikes have also attempted to adjust these brakes with no luck). it is only when i get on the bike and actually ride it that the brakes rub. is it possible that i'm just too heavy for this type of bicycle? (approx 175lbs) can my weight on the seat be somehow causing the alignment to misadjust?

First check your rear wheel for excessive play. The fact that it spins fine when off the ground, but not once ridden makes me suspect your bearing/cone adjustment is off. With the wheel off the ground grab the wheel and wiggle it laterally to check for any looseness.

Also check the brake housing routing to the brake. If it's too short or too long, you can have such trouble.

First check your rear wheel for excessive play. The fact that it spins fine when off the ground, but not once ridden makes me suspect your bearing/cone adjustment is off. With the wheel off the ground grab the wheel and wiggle it laterally to check for any looseness.

Also check the brake housing routing to the brake. If it's too short or too long, you can have such trouble.

Also check that the wheel is seated properly in the dropouts. Put the bike on the ground, undo the quick release/nuts and then do them back up while the bike is on the ground.

Are the pads that close to the rims that sitting on the bike will cause them to move just by that small amount? If you can take some macro shots of your brake setup. One when the lever is depressed, one when it is not.

You've been back 3 times? There may be something going here that is beyond what we are hearing, but if any of the ideas suggested so far are the answer, I would be suggesting you need a better LBS.

How many miles have you done and did it do this right from purchase?

is it possible that i'm just too heavy for this type of bicycle? (approx 175lbs) can my weight on the seat be somehow causing the alignment to misadjust?

No, if the bike is in good working order, and there is no frame defect, it can support 250+ lbs.

Also inspect the wheel's spokes. Are they loose?

thank you all for your prompt & knowledgeable responses, i very much appreciate the help. finally i believe the problem has been identified. when my left leg is extended, it contacts the rear brake cable (as it exits the top tube braze-ons) in such a way as to put pressure on the calipers from one side only. (pix below) so that explains why it's only a problem when i'm on the bike! now, any further suggestions? i could switch to a side pull dual-pivot brake on the rear.... anybody seen this before, or have any ideas? again, thanks!

if your sure tat its you legs pressing against the cable moving it towards the seat tube ziptie the cable to the seat tube and adjust the centering screws so that the brakes are centered then take it out for a test ride and see what happens

Your finger is on what's called a noodle of linear pull (V) type brakes. They come in various bend angles and lengths. Try to match one up for your bike so that it clears your leg better.

you rock, that's a great suggestion. i'll hit my lbs tomorrow. glad to know also that it's sure not their fault they couldn't diagnose the problem!