[Tf175]

The cross member tanks mentioned - they are the " ping " tanks for the air bags on my '86 . I presume same on your 375 . If you manage to loosen the drains on them it will likely 
deflate the air bags nearest the respective tank . I know you have jack stands , but be careful . 
Your plans to isolate sections of the system is the way to go as Richard and others have pointed out . 
I would add , do what is necessary to find out if you can develop sufficient pressure in the first place . 
Creative plumbing .

[dannymarilyn]

Dear Jeff,

I'd pay freight for your old Gast compressor. I just rebuilt our's (#277, 1991) and appears to be
the first time it was ever touched.

I would recommend whichever compressor you have, ensure there is a GOOD, functioning check valve
on the output side. No compressor is built to be under pressure al the time, and certainly not during
startup.

All the best, Danny