Won't a bad cap still pull fluid back from the reservoir.
1) Get it hot and check for leaks.
2) Take it to shop and pressure test the system/cap/cylinders.
You should stop driving this truck as a bad headgasket could lead to motor failure. I had a gasket go on me, and it sucked all the coolant through the motor, then cooked the motor.
If the cap is bad it will push more fluid out that the reservoir can handle so you will loose fluid rather quickly out the overflow tube. I will suck what's left back into the radiator but it will seem as if the fluid is disapearing.
