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Wow............bet it really pulls to the RIGHT when you step on the brakes?? It should with the left brake line plugged off! Not a safe thing to leave alone. Likely either the caliper is frozen(or leaks), thus they capped it off. A quick fix if you're stuck out in nowhere land, but not good to leave that way.
Re-install the line and pump the brakes and see what happens. Fix what's wrong before driving it. Likely the Passenger side is near the same condition, so I'd look at it also.
Love the spring spacers and old school "car phone"(bet it might have been an intercom between truck and camper)! The large plug looks to be a power source for a previous slide in camper.