There are a large number of possible faults - some of which Joe mentions.
(1) The acceptable system leakage rate as specified in the Ford Crago workshop manual is 0.3 bar or 5 psi in 15 minutes (with system fully charged, engine stopped and parking brake apllied).
(2) Assuming your horsebox is not piped for towing (unlikely if it's an 0813), the time for the compressor to fully charge the system (i.e. to 11 bar) is 6 minutes and 3 minutes to 65 % Full charge pressure i.e. 7.2 bar).
So fully drain your air reservoirs (by pressing brake pedal with engine off) and then restart and note time it takes to get to 7.2 and 11 bar on pressure gauge (are both gauges reading the same?).
If its shorter than these times, it is within spec and there is nothing wrong with the compressor.
To answer your question, as long as you have approx. 100 psi your brakes will operate properly. The parking brake is spring applied and air released. If there is insufficient air pressure available at the actuator, the spring will apply the brake as the air pressure cannot hold it off. If this happens while driving it would clearly be a problem, possibly causing the truck to slew around. If it's a case that the truck is stationary and you don't have enough pressure to release the hand (parking) brake, there is a mechanical wind-off facility - i.e. you slacken the turnbuckle on the parking brake cable - using 2 spanners. So if you do get stuck somewhere - you can release the brakes!
JD
December 2008