Rob Kardashian & Blac Chyna Are On the Juice

Rob Kardashian and Blac Chyna look like their weekly cheat day has become cheat DAYS -- they started their day by pulling through a McDonald's drive-thru for breakfast.

No word if they got any hash browns (wouldn't blame 'em) ... but we can see they got OJ. Not the worst diet slipup, but the sugar could be an issue.

In the last month they've hit up a couple of In-N-Outs and even pulled over for ice cream on the side of the road.

As they say, quitting is easy ... when you've done it a hundred times.