Does it work every time from the front lever?? If so then the problem is as others have said the rear pressure switch. If not then read on...

I had a weird problem similar to yours. I was having intermittent brake light from both hand and foot brake. My rear bulb socket actually melted (YEAH MADE OUT OF PLASTIC) and was making intermittent contact with the bulb. Since it is mounted low and difficult to see I missed this at first. Then I went to the brake relay. The brake relay and starter relay (Not the solenoid) are the same so I swapped them. Still no brake light so I put the relays back where they belong ( or so I thought) and still no brake light but now the damn thing won't start either. OK I decided it was time to go buy the electrical manual and stop guessing at what wires were for what. Once I had the schematic I figured out that I put the starter relay in the wrong way. OK now that is fixed, the bike starts again. Finally traced the problem to the bad bulb socket and once that was replaced all worked as advertised. So I suggest you look at the socket for either corrosion or general condition. Also if you can pull the connector off the rear pressure switch and jump the connections. If the light works every time then it is the switch.

OK Since you are having trouble with BOTH I would hold off replacing the pressure switch right now. I HIGHLY suggest you get a schematic and see if there is a relay in the circuit. Since it fails from both front and back it is most likely something common to both thus the bulb, wiring, socket, relay or anything else common to both switches. (there is a small micro switch in the hand brake).

As for replacing the pressure switch you should not have to replace the line but you will have to bleed the brakes if you do replace it.