I had the same thing happen in mr R30 Skyline,
Do not use 2nd had ones ever (learned that the hard way)
I ended up buying a pair that were listed for the MQ L28 patrol, had to cut the ends but it worked perfectly.
I have blown 2 heater cores now, spent a Canberra winter without heat. Finally figured out what the problem is too.
I bought this nice shiny metallic blue CPC Radiator cap, cos it was newer (old one had cracks in teh rubber etc)
What I didnt think of is the PSI rating was higher than the factory setup, so instead of depressurising into the overflow bottle at 0.9 bar, it would keep building pressure to 1.5 bar and pop the heater core, or the heater hoses.
so now I have new radiator, all new hoses, new cap(at 0.9 bar) and still no heater (took 12 hours to swap in the last one).....