brakes locking up

'99 f-150, 5.4, 85k miles. Problem, the front brakes will LOCK !!!! if the brakes are damp. Not from running through water or while driving. This occurs after sitting, inside, outside no matter. All it takes if high humidity brakes do not have to be wet from driving through water.
If after sitting for while, overnight or a few hours with high humid conditions, pulling off at a slow speed, the brakes are just touched, THE FRONT BRAKES WILL LOCK UP TIGHT. not drag or grab but LOCK UP. If I drive up to maybe 30 to 40 mph and lightly apply the brakes, generating a little heat, the problem goes away until next time. It's been this way since not too long after I bought the truck new. With 85k miles the front pads have been replaced due to normal wear. Still, lock up problem hasn't changed. As I said even slight pressure at a speed where the front wheels can't lock so easily, and the problem goes away until next time we have rain or humid weather. This is with moisture absorbed by the pads and not from running through water. Fact is, it doesn't happen if I run through a FLOOD !!!!! It's got me stumped for sure. Hasn't been a serious problem since I found out how to "fix" it each time. ANYBODY HAVE AN IDEA WHAT MAY BE THE CLUPRIT HERE?
Brakes are well maintained by a reputable shop. Same one does my tires, alignment etc. They do not seem to have an idea and all the manuals I've read do not list a similar problem.
There has never been any grease on the pads and the truck has never been on a dirt road !!! I have never noticed any problem with pulling or any lack of rear braking.....except when this occurs. Can't tell anything about the rear brakes when the lock up occurs and there is no indication of any problem with the rear brakes.
GOT ME STUMPED FOR SURE.
PS guess I could move to the desert  
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I've always had an interest in winches, wrenches, and WENCHES

Make sure that the anchor pins are greased and not worn. This can cause a floating caliper to stick in a bind. You may also consider  changing the rubber section of the brake lines. The Master cylinder may be capable of pushing through a swollen hose yet the caliper has no way of pushing the fluid back thus releasing pressure on the calipers. The heat from the friction can be making the pads and the disc  expand and gain you a little clearance, which may explain the release after heating them up. The rubber hoses will break down on the inside and cause the problems that you are describing. Hope this helps. If not, you may do a little studying on the proportional valve or even the master cylinder.  

I appreciate the info will check out the brake lines. When I drive off after the truck sits (in the high humidity) the brakes do not drag. The problem only occurs first time I put on the brakes....while moving at slow speed. If I get up to a resonable speed (30 or so mph)then, apply SLIGHT pressure, that bit of heat will eliminate the problem, until the next exposure to high humidity. If I apply hard to pressure at this speed, the truck will almost stand on it's head. Only slight pressure first time, will heat pads, slightly, and the problem is gone !!!!!!! This happened with the original pads as well as the replacements when they were changed, Problem is identical.
When I say Lock Up, that's exactly what they do. Tires will slide on pavement.... so I have to be careful or it will eat my tires up. I learned to do the slight pressure at some speed to prevent the lock up and to clear the problem.
Strange for sure. I maintained large industrial hydraulic systems for years and taught industrial maintenance for Ga Adult, Technical Ed for ten years and as simple  at automotive brake systems are, I am stumped by this problem.  
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I've always had an interest in winches, wrenches, and WENCHES

I think what may happen is the moisture aggravates your condition. If the pads are already against the rotor to begin with it just makes them grab worse. Maybe while the line sits with zero pressure on it there is still ample pressure on the calipers. The touch of the brakes forces fluid through the damaged hoses forcing them open and the fluid can move back a little but when you let up they may slowly close up again.???? Just random thoughts.

One other thing that I have seen that caused front brakes to lock is the wrong master cylinder installed with the wrong rod length. You didn't mention any changes so I didn,t mention this.  

Nope no changes everything stock. I don't think everybody is hearing that the problem ONLY OCCURS when the humidity is high...... There is absolutely NO DRAG on the brakes any other time. Never never happens unless the humigity is very high. If jacked up the front wheels will spin with NO DRAG AT ALL. Also, it happens with the truck in forward or reverse. Only the first time the brakes are applied. The rotors are smooth and with no rust etc. I can drive through pretty deep water or go through a car wash and NO PROBLEMS....unless the truck sits for a while with just humid air. If it is raining, I can go somewhere and let it sit for 30 minutes and will have the problem first time I apply the brakes. This, after the brakes are dry from driving anywhere or back home and the rotors are normal heat when it is parked.
When I replaced the pads last year (normal maintenance) I thought the problem would clear up but everything was the same after the work was done. They were not worn out but thought I'd replace them just in case that could be the problem....NOT.!

It has been 82 degrees here (about 26-27C)and it will be about 34 in morning    maybe this will be our last cold weather for this year. Had trees blown down last night and many without power due to lines down. ENOUGH    
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I've always had an interest in winches, wrenches, and WENCHES

I have sorta have the same problem, except it's my rear drums in my my 01 Ranger. I come out of the mud, car wash, rain,...... Then they always stick and I look like an idiot when my rear locks up completely. I have no idea what it is. I know my Dakota used to do it sometimes if it sat for days while it rained, but never like this. Any ideas?  

This is no longer a problem. December 8th my truck caught fire due to defective cruise cut off switch, totalled out truck, much of my house and almost got me with it. P-Oed about the loss of my truck for sure. "I LOVED MY TRUCK"  
I didn't receive the call back about this defective switch. there have been thousands of ford products with some lives lost due to this switch. Our local fire dept even had one catch fire in the FIRE STATION for same reason.
Check with your dealer if you have ANY FORD last 15 years, may save your life, house or truck.
Just got back in my rebuilt house April 14th. like, better than new. Saved most of contents. Thank God I live within 1/4 mile of fire station and had THE HARTFORD insurance on house.  
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I've always had an interest in winches, wrenches, and WENCHES

Fords of this era always lock up brakes in high humidity usually it is only in the front. on the rear it usually is contaminated brake shoes. especially on bonded linings sometimes the glue used to attach the friction material seeps through and causes lockups. switch to quality brake shoes (riveted not glued) and drums and your problem should go away.