225 is a great engine. I have seen many with extremely low pressure at idle and last forever. I agree with putting on a mechanical gauge!
You will learn a lot with that project. 225's are odd fire engines. If original it would have the 3 speed and Spicer 18 transfercase and the offset D44 rearend. Those are good little jeeps!
1984 CJ7, 258..HEI..MC2100, T176/D300, D30/M20 lockers, lift, warn winch!
"When in doubt! Gas it out!"