To remove lead deposits on top of the mould blocks or on the mould block faces:
If the mould is iron, get the mould HOT. I recommend that you sit the mould on molten metal (you'll have to prop it up) for five minutes. Now, the mould block will be the temp of the molten metal. Then, while it is still hot, take a fine brass "toothbrush" and scrub the lead off. This will clean the mould of ALL debris.
If the mould is aluminum, heat it up (two minutes sitting on lead should be enough) and scrub with a rough cloth (be sure it is 100% natural fiber - wool is good as is burlap). I mean SCRUB it!
This will remove the most stubborn deposits. I have, in the past, covered the mould surfaces with Rapine Mould Release. Jim Borton suggested a grahpite spray but I have yet to locate it at any hardware or auto parts store and I HAVE looked. If I can find that I would be happy to try it.
Recently, on the "Cast Boolit" forum (spelling is correct) I saw a suggestion to use "Bullshop Bull Plate Sprue Plate Lube":
(You need to Login or Register to view media files and links) Be sure and follow the directions. It is the best thing I have tried for this purpose. Any lead deposited on unwanted mould surfaces can be easily wiped off while using the mould with a que tip (I often just rub a gloved finger over it and it disappears). This is great stuff for the bullet caster.
Dale53