<p>Late last month, the nation&rsquo;s largest pension fund, the California Public Employees&rsquo; Retirement System, rolled back the expected rate of return on its investments, bracing for more conservative gains after a bull-market run pushed stocks to record highs.</p> <p>The decision means that the system&rsquo;s 3,000 cities, counties, school districts and other public agencies will have to put more taxpayer money into the fund because they can&rsquo;t count as heavily on expected investment income to cover future benefit checks. The impact will be the biggest for those already deeply in debt to the retirement plan and struggling to catch up.</p>