Intermittent operation;
If CC seems to work for awhile after starting then stops after the engine bay gets up to temperature, check the master cylinder pressure switch for being open when the trouble is in..
Pull the plug off and test with an Ohm meter.
The issue is a switch that is too sensitive to pressure.
Brake fluid expands with heat and can operate the switch.
This would prevent 12 volts from going to the CC unit.
CC electronics almost never go faulty.
A factory/dealer recall has been in effect for some years due to a leaking pressure switch that could cause fire at 'anytime' truck in use or not.
It is a harness that puts a small value fuse in line to blow should leakage develop causing a fire.

 

Continuing education;
The first replacement sensor failed and was found temperature sensitive dependent.
Now the second switch is failing in these high ambient temps over 90 degrees plus engine bay heat the same way especially after shutdown and engine bay heat soak.
Research shows the switch is to open it's contacts on 5 to 10 psi of master cylinder fluid pressure.
Without specific testing of the switch to check it's sensitivity, it is unknown if the switch is mfg to spec and/or the master cylinder pressure rises (unactuated)from fluid expansion that causes the sensor to open it's contacts until the temperature fall to some lower point while driving.
Will update as the issue is worked out.

 

Bluegrass,
Expanding brake fluid or not, there should be no residual pressure in the master cylinder when brakes are not applied.

First movement forward of the booster's output rod into the master cylinder moves the pistons a small amount to close the Compensation Ports. The C-Ps equalize pressure between system and reservoir. Once C-Ps are closed, further movement of pistons forward builds system pressure. When foot comes off of brake, pedal should retract all the way, allowing booster input, and therefore booster output rod, to release master's pistons to the position that the C-Ps are open.

Just some thoughts:
Any chance that the pedal assembly is hanging up some, not allowing pedal to release all the way?
Or cap on the master cylinder is not venting to the atmosphere?
Or the booster output rod is not retracting as far as it should?
Or the master cylinder is hanging up and not allowing full rearward return of its pistons?

The pressure switch on the master opens/closes power to the electrical clutch that allows the muscle to move the servo output. Idea being that in case of a failure of the BOO switch, that the brake system pressure when activated would disengage the servo as a safety. In effect, an electrical analogy to the old vacuum dump valve at the brake pedal on the old all-vacuum cruise control.

The fuse in the safety harness that was added in series for the fire recall is a 2 Amp. Idea being that the clutch required less than 2 Amps, and if the pressure switch ever leaked again, hopefully the 2 Amp fuse would blow before much heat was created due to the leakage current path.

 

Thank you Torky.
I know all this.
My issue is temperature.
Above a certain temperature the sensor switch opens from residual pressure in the master.
I can drive and have the CC work but when I come to restart the motor after it heat soaks the engine bay the CC does not work again until the bay cools down again providing the outside temp is low enough.
Brake fluid does expand to some degree same as transmission fluid.
The front pads are always in contact to the front discs with light pressure.
Granted there may be an issue with pressure equalization but I have not proven that yet.
The original sensor leaked and was replaces with a new unit.
A short time later the problem began until I saw it was temperature sensitive.
Replace with a second sensor and the same issue prevails.
It was over 95 today, made a change and drove 140 miles to see what happens and it appears to have helped greatly.
The change has almost cleared the issue.
I'm not done by any means with the problem.
An internet research of other board shows a lot of owners have the issue but no one knows what all is going on.
All they do it change the sensor and see it good until the problem returns again.
It may be a sensor not made to specs, it may be heat generated pressure opening the sensor or it may be a combination of both or a master cylinder issue.
Until it's all gone through the root cause is still unknown.
I would test pressure with a gauge but the problem with that is the system pressure can get upwards of 1200 psi.
Trying to see 5 psi accurately on a 1500 psi gauge would be very difficult.
It may turn out to be just pressure imbalance.
The armored lines go straight down to the ABS unit and are subject to a lot of exhaust manifold heat.
I'm not saying your wrong about anything but it has to be worked through to find the root cause.
Thanks again.
Good luck.

 

Conclusion:
After the summer temps returned hot again, the third replacement master cylinder sensor switch was replaced with a part form Ford dealer.
There has been no more issues form heat.
Problem is cleared up.
The after market sensors from one source are manufactured to too low an operating pressure spec.
The brake system fluid expands with engine bay heat and higher ambient temps just enough the cause the after market part to open it's contact causing the Cruise Control to be powered down and unable to operate until the temperature cools such as at night or traveling to an area that is cooler.
I detected this action over several months and noted the temperature effects.
Good luck.

 

Hello, I'm asking on behalf of my pal who just bought a '97 F-150. Where is this master cylinder pressure switch located? Does removing it create the need to bleed the system? I'm hooking up with him in a few minutes to change the differential fluids. Thanks. Kira

 

May be a bit late on this but the switch is right in the end of the master cylinder along side the fuse box..
Have your new switch ready to put in as soon as you remove the old.
Only thing you will see is the fluid will begin to bleed out the hole, so no brake bleeding needs to be done.
Good luck.

 

LOL....I just pulled the connector off the pressure switch and installed a jumper until I got the switch fixed....that was 5 years ago...LOL, I'll get to it. The cruise still works fine. I assume that must be wired in series (N/C) with the BOO switch. I DO NOT RECCOMEND THIS AS A FIX...

 

The switch provides 12 volts to the cruise control unit and cut it under pressure rise.
It is one of two or more fast kick-off safeties mandated by law.
The BOO knock-off 'applies' battery to the cruise control.
One action takes battery off, one applies it as an inhibit signal for fast action assurance the controller stops operating.
The third is the off switch on the steering wheel.
The control switches on the right side act to slow to be any real safety when the unit is on and working.
Good luck.