A/C: Why does the LOW-side pressure *increase* when the compressor runs?

My heater badly needed replacement (leaking a fine mist of coolant inside the cabin), the A/C hadn’t held a charge for years, and they’re both in the same box that’s a pain to get to and you have to break both systems to get the box out. So I replaced both at the same time. New heater core, and a completely new A/C system except for one hose that I didn’t realize was absent from both kits that I thought would cover everything. Oh well, it’s just an open hose, nothing special about it so long as it doesn’t leak.

Anyway, I put it all together, vacuumed it out, put the first can on, and…what?!? BOTH gauges spike way up when the compressor engages, and quickly fall back down when it stops. Different angle for each needle because of the different scales, but the same pressure always. Perfect tracking between them.

Thinking through how the plumbing works, I figured there must be a plug somewhere between the low-pressure port and the compressor. The orifice tube becomes moot because there’s no flow, so both sides have the same pressure. So I took it back apart, immediately downstream of the low-pressure port and off of the compressor, and blew some dry shop air into it, expecting nothing from the other end. But I got a lot! In both directions! And I didn’t see anything blocking the compressor’s input either, like maybe a plastic dust cover that I forgot to take out. Nope, that’s open too.

Okay, so the new evaporator and dryer are good, which means I don’t have to dig the box back out again. YAY! But I still have this problem. Why do BOTH sides increase, exactly the same, when the compressor runs?