I've had my 87 300D since november and I'm now getting the indicator light on my dash that's telling me I should get the brake pads checked. So I took it to my very very trusted mechanic (Seriously, he's always been a great mechanic). He told my father in a phone call that I still had 30% of the pads left. My father told me you don't need to get them changed since they're still good. Now, I tust the Mechanic, and I trust my Father, but something about a bright light on my dash that lights up each time I break disturbs me. Does anyone know of a guide that'll show me what I need to do to check the breaks and replace them if neccesary?

If your sensors are correct, then you should get your pads replaced soon. It's not critical right now, it's just telling you that your pads are getting low. I find that under normal driving conditions you have around 1,000 miles till your metal against metal. You really should get your pads replaced ASAP however because if you over-extend your brake piston, the tech will not get it back in when replacing your pads and your looking at a nice set of 300.00 brake calipers.

When the brake pad indicator first comes on I've found that that usually means you have around 10-15% left on your pads, not 30%. Like talonlover said, get the pads replaced soon before you damage the rotors as well. :)