Spartan Dash A/C

[3rd stage]

I'm about to add more R-134 to my Beaver's dash A/C system, but I'm confused. There are 2 low pressure fill ports- one at the compressor in the rear, and one at the evaporator behind the dash. Static pressure at the front port (system not running) is around 50lbs, but at the rear one it's 10lbs or less. These are both definitely low pressure ports. The single high pressure port is at the compressor.
Firstly, why are there 2 low pressure fill ports, and secondly, if both ports are on the low pressure side, how can the pressures be different?
I know a little about A/C (enough to be dangerous, I guess!!!), but this has me baffled.
Thanks for any help.
Andy

[zzzs]

Andy
My Beaver is a 2004 Marquis and when my A/C needed recharging, the A/C guy could only find the low side port on the drivers side firewall. So he installed a high and low port at the compressor and recharged the system from there. We contacted BCS in Bend Oregon and they use 4.5 lbs of R-134, they also said Beaver only installed the low port and didn't measure pressure because of the long refrigerant lines to the condenser giving erratic pressure measurements.

[3rd stage]

Thanks, ZZZS.
Any idea why the pressure readings between the two low pressure ports are different? Is this the "erratic readings" that you were referring to?
Thanks.
Andy

[a1elect]

changing from a gas to a liquid and 40' of line would be my geuss.