A/C not accepting Freon

I replaced my low pressure line, because it was cracked at the elbow. I hooked up the vacuum and manifold and vacuumed it down to around 30 in of mercury. I cranked the truck and turned on the a/c. However, when I opened up the valve to let the freon in, it only flowed for a second and stopped flowing. The compressor kicked on and the pressure built to 40 PSI on the low side and around 50 PSI on the high, but it only sucked out about 4oz of Freon (freon can setting on a digital scale) and wont take any more.

Why would it only take 4oz and stop?