Leaky Rad Cap

89 4Runner, just did Valve cover gasket, pcv, egr, intake, plugs wires, cap and rotor. Comes up to temperature , pressure in the system and leaking coolant from the rad cap. Water pumps and thermostat are good, rad cap checked and holds 13psi and blows out at 15psi.

Rear heater core may be no good, will this cause any problems?

When you say it leaks, does it come out of the overflow tube or does it weep past the cap's seal and down the radiator itself?

The overflow tube leak is the system exceeding the cap's pressure rating. If the rear core is plugged (answered in your other thread) then it may increase the pressure of the system to the point of release by the cap. This is probably because your front and rear heater cores circulate coolant in parallel...a blockage in one won't prevent heat to the other. However, blocking half the flow path will cause an increase in pressure in the other half because the water pump is trying to pump twice the coolant through the one core.

If the cap is leaking from the outer seal over the radiator, then the sealing surface of the radiator may need to be smoothed out or the cap isn't sealing properly.

Perfect, thank you thank you. My coolant was just weeping out the top of the radiator from the cap and running down the rad itself. I tightened the cap down on the rad and idled up to temperature, filled the overflow and it worked just fine. I will watch to see if I leak out the overflow now.

There is a prestone plastic cap at the back of the engine behind the intake manifold mounted on the fire wall. Is that the rear core? If so there was coolant in it and loosing the cap pushed coolant out the rad with no cap on the rad and idling. So does that mean it's flowing to the rear?

The rear cap is the one to use for filling the system. It is the highest point in the engine cooling system. Fill the radiator first through the radiator cap while the rear cap is also off. Replace the cap on the radiator. Keep the cap on the radiator and fill the cap in the rear while hot and both the front and rear heaters are on, and then close it. Fill the reservoir to the hot level and you should be good.