Agree with Ed: on my 2310- I press down (takes a fair amount of pressure) while slowly moving to engage diff lock. Only use when necessary; it will eventually unlock itself after you've taken your foot off it. I rarely need it, in fact have only used it a few times when I was "stuck." It worked well in those cases. Otherwise I just use 4WD.
Haven't had to deal with poor brakes, but there have been a few instances of the brake lock letting go; moral, NEVER park on a slope, especially not with engine idling. If you have to stop to park it for a bit, first get it level or at least cross-slope, not pointed downhill.
Also, these older models have a different key set-up: there's no indicator light for them, but to activate glow plugs, turn key to the left (it's spring loaded). I generally have to hold mine for at least 10-15 seconds before the glow plugs begin to amount to much. Often I'll hold the key in the left position for almost a minute, maybe repeat if it's really cold and I haven't had my block heater plugged in long enough. When done with glow plugs, turn key to the right for the usual 3-position OFF-ON-START (spring-loaded) use.
Does yours have turf tires or other type? Mine has turfs, and they've worked out well.
And yes, read that manual --STUDY that manual-- when you get it. These are great little tractors!