What is the next step

[firefly]

my mate has fitted the thermostat, i managed the rad cap all on my own now i tested the old thermostat, and it opened up like a gud un when the pan of water started to boil soooooooooooo , if it int the cap or the stat and the head is ok and not losing pressure what could it be thats making the truck lose water

[The Lang-Shankit Scunner]

Leak. Try pipes to and from metal ones, and chesk by the heater hoses, that goes up to the bulkhead, and check for wet carpets from heater matrix.

Check head again because some have had symptoms of water loss only.

[Tallyman]

The thermostat should open before the water gets to boiling point. It should open at around 88 degrees if it's the standard one. If it's only opening when it gets to boiling point (100 degrees) then it's not working correctly.

[marky]

Check around the WATER PUMP as well