When Congress passed the 1976 Tax Reform Act last autumn, the bill was hailed by sponsors as a major crackdown on abuses of the tax system. The massive legislation contained some 250 separate provisions. Many of them were long-sought-after "reforms."

Tax experts were realistic about the measure. It was obvious the new law wouldn't close all the "loopholes" liberals had wanted. But it would crack down substantially on the worst of the offenders - the phony tax shelters that allowed wealthy persons to escape payment of taxes.

Rep. Al Ullman (D-Ore.), chairman of the House Ways and means Committee, called the new legislation a major step toward greater equity in the tax system. Even some longtime "tax reform" advocates had hopes the measure would close some of the main avenues for abuse.

Now, a little more than a year after the 1976 changes were enacted, the results are beginning to come in and the verdict is a mixed one. From a legal stand-point, the tax-shelter crackdown has seriously limited the number of legitimate tax dodges available to rich persons.

But it's also clear that it may take several more years for the changes really to take hold. Judging from initial reports, experts say Internal Revenue Service enforces may have a big job to do next year to convince some tax-payers the 1976 act meant business.

Last year's legislation was aimed mainly at curbing the more dubious tax-shelter schemes being peddled by promoters. The deals would bring together dozens of rich investors in a limited partnership, use their credit to borrow money, and offer them a "paper" tax loss of virtually no risk.

The schemes worked primarily because such limited partnerships qualified for non-recourse loans. Under these arrangements. If the deal later fell through, the partnership would be liable for the loan but the individual investors couldn't be damned. A near-perfect tax-dodge.

The 1976 legislation sought to crack down on tax-shelter abuse in two ways.

First, it limited the deductions a taxpayer could take under most of these schemes to the actual amount of money he was risking on the venture himself - apart from any non-recourse loans that might be made available to the partnership.

Second, it broadened the scope of the so-called "minimum tax" - a sort of catch-all tax provision designed to force payment of some taxes on tax-shelter investments. The minimum tax has been moderately successful in preventing wealthy persons from escaping all taxes - but not foolproof.

The crackdown didn't hit as hard at the biggest tax-shelter of the bunch - real-estate syndicates. In the case of real-estate, investors still may take the same deduction as before, but instead of writing it off a single year, they now have to amortize it over several years.

Still, the law did move to close off (or reduce) most of the other major abuses - ventures involving oil and gas drilling, absentee farming, sports franchises, motion picture production and equipment-leasing.

To be sure, congressional experts say it's still a little early to make a definitive assessment of the 1976 crackdown. Most of the new restrictions weren't even slated to take effect until last January 1. The impact won't show up on IRS tax returns until after next April.

Moreover, the IRS hasn't even begun writing formal regulations needed to administer the 1976 act. So tax lawyers are making decisions based on their own interpretation. And as the very existence of tax lawyers attests, the law isn't always clear simply by itself.

But tax experts in the field say they've had enough experience with taxpayers' behavior in the wake of the 1976 act to come up with a few preliminary conclusions about how it's working in its first year of operation. And the outlook appears to be mixed.

Lawyers say on one hand, the new act seems "pretty well to have killed" the specific tax-shelter partnership schemes it was designed to close off. (One sign is the vigorous complaining heard now from motion-picture producers. The money just isn't there any more, they say.)

At the same time, however, tax experts report two new trends:

Investments in real estate, the one shelter that emerged relatively unscathed from the 1976 crackdown, have increased dramatically - a development many tax lawyers (and some economists) say is a major factor in sustaining the current housing boom.

Promoters who sell tax-shelter schemes have begun packaging a plethora of "new" deals to replace those cut back by the 1976 legislation - many of which tax lawyers say are dubious or outright illegal. Despite warnings by some attorneys, many taxpayers seem willing to take the chance.

Tax lawyers say one favorite new tax-dodge device is the establishment of a limited partnership to lease reserch-and-development "services" to large corporations. The scheme avoids, technically at least, some of the pitfalls of the other shelters, while still offering a tax "loss."

As such, says Martin Ginsburg, a New York tax lawyer who keeps tabs on tax-shelter investment, "at best you have a Scotch verdict" on the 1976 act. Howard Krane, a Chicago tax attorney, agrees. Says one tax expert here: "It's not as effective as we hoped. There's a lot of game-playing going on."

The question is, how effectively will IRS be able to enforce the new legislation? Although the agency's new commissioner, Jerome Kurtz, has promised a doubling of IRS's auditing next year, that still will double-check only a small percentage of total returns.

Treasury officials would like to seek statutory authority for IRS to audit limited partnerships more thoroughly, but that, too, seems unlikely to happen soon.

In the meantime, first reports seem to show that while the 1976 act was a step toward "tax equity," it was only a modest one. Full "reform" of the nation's tangled tax code - and a hard look at present tax breaks - still awaits action by the President and Congress.