1999 Chevy Silverado wheel speed module

Sometime at 500 rpm or a little least, I am loosing my power brake. Could it be that one of the swheel speed module is gone. How much to replace it
Or what would the cause for this malfunction. Best regards Pierre

updated5/1/10
Alo Bob, Ken, I di what you have asked me cto do. I check the brake booster with a vaccuum pomp and \i tested it at 3, 5, 10, 15, and 20 inches and the booster was holding and no drop in vaccuum. Also check my check valve and when I took it off, there was a pressure so it is fonctioning properly. Now this problem is intermitante and does not occur every time I stop but only sometime when I back up or make some stop at low speed let;s say about 1 to 3 miles per hour. My rpm gage read between at iddle 500-700 rpm and when I put the trasnsmission in drive then itdrop down to 500 and sometime a littlelower then 450 rpm so what could be causing this problem. Could the dealerhook up a cpmputer and could the computer read the reason for this malfuctionning? Well awaiting an answer or guidance from you Best regards, Pierre