Former defense secretary Harold Brown contends that President Reagan will waste up to $30 billion if he goes ahead with building an updated B1 bomber rather than waiting for the radar-evading Stealth aircraft to be perfected.

In a meeting with reporters Friday, Brown warned that the Soviet Union has been working for years on defenses for knocking down the B1 bomber. If Reagan does the expected and goes ahead with the B1 this year, those defenses will be deadly by the time the B1 flies in the late 1980s or early 1990s, the former secretary predicted.

Breaking his self-imposed public silence on defense issues during a breakfast meeting with Pentagon reporters, Brown scoffed at the Air Force argument that the B1 would be valuable for conventional bombing in remote areas like the Persian Gulf.

"What you're saying is that you're going to spend 15 billion to 20 or 30 billion to be able to bomb the Bulgarians." He said the United States already has plenty of aircraft for conventional bombing where defenses are unsophisticated.

If he were back in office, said former president Carter's defense secretary, he would recommend building the Stealth bomber to force the Soviets to invest in a whole new network of air defenses, taking money away from other military programs more menacing to the United States and NATO allies.

As for building the B1 to penetrate Soviet airspace to deliver nuclear weapons in a war, Brown argued this is not "even the second best choice." Cruise missiles launched from outside the Soviet Union and the Stealth bomber would be much better investments than the B1.

The Air Force has recommended starting to build a fleet of B1s right away while work on the Stealth continues at a fast pace. The Pentagon could buy 50 or 100 B1s, depending how well Stealth comes along.

Brown said shutting off the B1 is easier said than done because Congress keeps big weapons programs going, whether the Pentagon wants them or not, to protect jobs in lawmakers' home areas.

The former defense secretary, a physicist who helped develop the hydrogen bomb and has spent his adult life wrestling with arms control problems, made these other points during the breakfast session with reporters:

Missile accuracy: Both the United States and Soviet Union have developed guidance systems that can achieve, beyond a reasonable doubt, pinpoint accuracy on missiles fired at each other. He said satellites flying in various orbits have supplied the necessary information on varying gravitational pulls under the missile flight paths. If either side doubted its missile accuracy, he added, it could put guidance in the warhead itself to enable it to hit a silo cover. The space ship Voyager's flight to Saturn demonstrated that guidance is indeed in hand to put a missile within 30 feet of the target after flying 5,000 miles. "You can be that accurate."

Anti-ballistic missiles: The fatal flaw in these systems is that the attacker can use up the defending missiles by showering them with too many warheads to handle. Therefore, relying on an ABM system alone to protect land missiles standing underground continues to be a losing proposition. "The only breakthroughs that have taken place in these last eight months are public relations breakthroughs."

MX land missile: Moving this new missile from one shelter to another to foil Soviet gunners trying to target them is still the best way to preserve the land leg of the strategic triad consisting of land-based missiles, bombers and submarine missiles.

AWACs warning planes to Saudia Arabia. "I don't think there's a major military threat to Israel. Israeli objections are as much political as they are military."

Arms talks: The best way for the administration to get talks back on track is to broaden those promised on theater nuclear weapons to also include strategic arms. He said there is no prospect that either the United States or Soviet Union will achieve and hold superiority in nuclear weapons, making negotiated arms reductions the best hope for slowing down this race and avoiding mutual destruction in a no-win nuclear war.

Pro-defense consensus: "My greatest fear" is that national agreement on the need to beef up defenses will be shattered as the administration and Congress cut the Pentagon budget to help the national economy. "The solution may be to increase taxes only for defense."