I don't know about your car, but on our 90 740 there is sensor in the master cylinder resevoir that causes the brake failure light to turn on if the fluid gets low - in fact it just did that last week. I was surprised that the fluid level doesn't have to drop much for the light to turn on.

If there are a couple wires connected to the top of the resevoir, than you've got that sensor also. Make sure the fluid level is topped off regardless.

OK - I just looked at the FCP Groton site and according to that, the 84 doesn't have the sensor ($55.00).....

http://www.fcpgroton.com/images/072-1282.jpg

They show the later years with ABS HAVE the sensor ($109.00).....

http://www.fcpgroton.com/images/3530972.jpg

Given the price of $55.00 for the part, plus a few bucks for fluid, I would think $350 is a bit pricey. It's hard to imagine more than a couple hours labor, which even at $60/hr would bring the total up to the $180-200 range.

Ask them to justify the 350 - X for the part, Y for the labor. It might be that they are being honest on the labor and at the same time some parts house is giving them an outrageous quote for the parts. More likely the mechanic is marking up the price for the part.