Make sure the cap is good and it's the correct cap. I went through this a couple months ago. Also, make sure that the overflow line to the catch tank isn't plugged. I spent a week fighting with caps only to figure out (finally) that the line was plugged.

Edward

'97 Montero Sport LS 5-Speed 3.5L conversion
SAS Dana 44s & ARBs, 35" Yoko Geolandar M/Ts
NP231 B4R doubler/Terra Low231/RP 5.38 229:1
'99 Montero Sport Limited 4WD SAS 3-link project
'03 Montero Sport Limited AWD
'97 Montero Sport LS 5-Speed 4WD