Reddit mentions of Ruger Single Action Revolvers Blackhawk, Vaquero, and Bisley Disassembly & Reassembly Gun-guide (Disassembly & Reassembly Guide)

The hammer lock is in the sear; but it also engages the indexing pawl and the cylinder latch - and both of these are exposed when you are doing this work. I would suggest you start there and work your way inward. If I were a betting man, I would place my money on the cylinder latch spring being slightly out of alignment (probably not seating all the way down) with the cylinder latch and not allowing lock-up to occur, although the pawl has much the same effect, and is potentially damaged easily while reassembling the pistol. Another potential problem is if the base pin isn't fully engaged, the mechanism won't work correctly either. This is the easiest to fix (just try to seat the base pin further into the revolver) so try it first. The stronger Base Pin Latch spring can make it a little tougher to seat the base pin correctly; but once it's seated it's locked!

If you don't have a copy of [Ruger Single Action Revolvers Blackhawk, Vaquero, and Bisley Disassembly & Reassembly Gun-guide (Disassembly & Reassembly Guide)]
(http://www.amazon.com/Revolvers-Blackhawk-Disassembly-Reassembly-Gun-guide/dp/B000MXSON0/ref=sr_1_9?ie=UTF8&qid=1415304577&sr=8-9&keywords=ruger+single+action+revolver) it's worth every penny of the $7 it costs from Amazon.

Good luck, and let me know if you get it figured out!