You need at least 100 psi to operate the brakes effectively.
The low pressure warning buzzer will come on when you start up if pressure is less than about 70 psi and will only stop buzzing above 70 psi. The brakes won't release if the pressure in the reservoirs has not built up.
The fact that both gauges have the same , low reading suggests:
( and this is a quote from the Ford Cargo workshop Manual)
"(a) Air leak in supply circuit from compressor.
(b) Faulty air limiter valve.
(c) Faulty safety valve - blowing off continuously.
(d) Restriction in compressor output circuit.
(e)Restruction in compressor air intake.
(f) Faulty compressor."
JD
March 2008