How to parse node data?

Are you posting json data? Or binary data? In both cases the data_plain field should contain the data, in your example it looks like there are a lot of zeros and one 1 so you are probably not sending the data you expect.

Take this example:
"data_raw": "gAQpAQIACQAB+Xuwi1yr5A==",
"data": "VCQ0",
"data_plain": "T$4",

Here you can see data_plain contains a couple of bytes of 'real' data (binary data so it does not make sense translated to ASCII)