We are experiencing a bit strange problem of single disc (mark-1) brakes not holding B-model aircraft at and above cross-over setting on all four engines. It is happening both in case of parking and pedal pressure braking. It is understood that single disc brakes were designed for T56A7 engines having take-off TIT of 970 whereas subject B-model is fitted with T56A15 engines having take-off TIT of 1085 (thus more torque). Brake assemblies, brake control valves, brake selector valves, brake accumulators, shuttle valves, relief valve have been replaced. Brake pressures have been found normal (1700-1800 psi in case of pedal pressure and 1500-1600 psi in case of parking brake). Thorough bleeding has been carried out to purge any air trapped in the system. Hydraulic plumbing / mechanical connections have been found satisfactory. Though anti-skid system is inoperative in case of parking brake or till aircraft is stationary, however, both anti-skid valves and control box have also been replaced. Anti-skid tester (hytrol tester) results have been found satisfactory. Subject B-model aircraft weight is 172000 excluding 28000 lbs of fuel. Both parking and pedal pressure brakes give abnormal noise and start releasing at all four engines at or above cross over thus aircraft start moving. Ground check out valve rigging has been found satisfactory as hydraulic is not shifting from one system to the other. Moreover, response of warm brakes has been found better than cold brakes. Guidance to fix the problem and sharing of similar experiences would be highly appreciated.