Adding CYA and recurring algae bloom

My pool is an in-ground 18500 gal with 3/4 HP pump(flow into pool ~40 gpm) and a sand filter. (Built in 1969 and have replaced liners several times). Been using the BBB method since Nov, 2010 and testing is done with TF-100 kit. Prior to that I used pucks, algaecide etc. When tested by pool company CYA was always around 50 and my evaporation rate was 3/8-1.2"

Currently evaporation rate is around 1/2" in 24 hrs or 160 gal/day. In 30 days will have replaced 4800 gals of fresh water= to 26% of total volume of pool. July 22, 2011 CYA had dropped to around 18. Purchased Instant conditioner and put in 52 oz per bottle instructions. Had to add 76 more oz before measurable increase to 30 overnight and 3 days later dropped to 25. My current reading (8/21/11) shows slightly less than 20.
I have been maintaining FC @ 2.5 to 3 but have a recurring slight green algae bloom every 2 weeks and have shocked to 60% CYA.
So a) do you think 26% water replacement enough to lower CYA 5-10 points? Do I face a monthly conditioner increase or go back to using pucks? and b) your thoughts on my inability to kill algae? My shock treatments took 3 days before the FC change is 0 overnight and my CC readings have been less than 0.2 consistently.

Hope the above is adequate for a reply.
ralph

The consistant 1/2" loss per day is high. I get that and more here in northeast PA but ONLY when weather conditions are just right. Low relative humidity, warm water, cool night, and a breeze. I think you have a small leak somewhere. Could be joints in the real old pluming but may be the liner or one of the penetrations or skimmer or stairs. In other words, anything. If you're familiar with the bucket test you can do that. I've done it with a roasting pan, maybe 5" deep or so, fill it so it just floats, loosely tie is in place to hold it in the center of the pool, then watch the water loss in it compared to the pool. By keeping it in the center of the pool you keep the pans water at the same temperature as the pool water and keeping it in the center of the pool and low in the water presents it with about the same wind conditions. If you do find you have a leak then post in the 'construction and repair' forum and we'll take it from there. As for the drop in CYA, yes, it is just a dilution calculation. If your CYA is 50 and you replace half the water, the CYA will then be 25. As for the algae questions, I'll let one of the others handle that.
Hope this helps.
Al
Edit: I just saw your thread about Fix-A-Leak. Since you already did the bucket test maybe you are just dealing with high evaporation. To get better measurement accuracy you can use a ruler and measure the water lever above some known point on a step. In calm water you should be able to read 1/16" with a little care.

I tested the skimmer yesterday morning (shut off the skimmer return and used green dye to check for leaks). I had a small 1/4" hairline crack and used a two part underwater pool epoxy to fix. After allowing couple hours for epoxy to set, turned skimmer back on and proceeded to do bucket test. This morning the pool level had dropped 1/8", bucket 3/16". Problem solved!!

Now how're you gonna fix that bucket?