firstly do you have springbrakes on the front axle [steer] or not?. whether on front or rear check pressure on park brake relay port 1 [should be over 8 bar system full] put gauge on port 4 of the relay and release handbrake and see how long it takes the gauge to go up to the same pressure as port 1 [should be within 1-2 sec]. if it takes longer there is a blockage or kinked line or possibly faulty hand brake valve. If it is quick then the problem lies from the delivery side [port2] of the park brake relay to the spring brakes. The spring brakes themselves only begin to release between 5,5 and 6 bar so your system pressure needs to be at normal operating level. If you don’t come right pm me and I will call you and help