Disappearing coolant

[91Rangerxlt21]

Truck info:
91 Ranger XLT 4x4
5 speed
2.9L v6

originally I had this issue and found it was leaking into my oil so I replaced the head gaskets and had the heads checked out they are okay and not cracked before reinstall. All new hardware ie gaskets headbolts oil(change 2x to flush the engine) new coolant the works. Everything seemed fine until today now I have the coolant disappearing again but it's not going in the oil it's not spilling out of any hoses checked the valve in the radiator and that is okay and not leaking there is no visible leaks that I can see. Though the radiator never goes completely empty it will go down to a 1/4 full and that truck has an overheated and the temperature gauge never goes above half and there's no fluctuation of up and down in the gauge indicating it's it's overheating. Someone mentioned that possibly the water pump is leaking it but I don't see any visible leaks from that area. I've even left cardboard underneath the truck to see if it would drip any on there that's not doing that either. Also there is no white smoke coming out of my exhaust. Going to get a pressure tester tomorrow to test the radiator pressure. The coolant system was flushed but I don't know if it was flush too well enough because the coolant does look kind of nasty now but again my oil is clean an clear. I went to an orv park today and have had to put about 3gal of water/coolant in it with in a 6-7hr period. Anytime thoughts ideas are Much appreciated an welcome.

 

[RonD]

And oil level is staying the same?

Best bet is to rent a cooling system pressure test kit
These have a solid radiator cap with a hand pump and pressure gauge on it
You put it on a Cold engine and pump up pressure to 20psi, engine normally runs at 14-16psi

Then watch for pressure drop, a leak, and since engine is not hot any leaking coolant will drip down to the ground, not evaporate on a hot surface
And you can safely feel around for the source of the leak

Check with auto parts stores, they often "loan" them, no charge, but with a deposit

EDIT:
OOPS, I see you are going to do that

 

[91Rangerxlt21]

Yes the oil level has stayed the same so far. Been keeping an eye on that. And okay thank you for the info on what to test the system to. Another question what if I cant find any leaks but lose pressure still?

 

[RonD]

Not possible

Check passenger side floor board for leaking heater core, if nothing in engine bay shows up