If the brakes initially hold and then are apparently releasing, without any fluid leakage to the atmosphere, what else can it be other than the roll control solenoid leaking? I'd put the vehicle on a lift and with a helper setting the roll control, see how much resistance to turning the front wheels have over a period of time. If they initially can't be turned, and then they can be turned with force, and then turned easily, I'd say you are losing your hydraulic pressure and the only way that can happen, if fluid isn't running on the ground, is through a solenoid leak.