The startling success of the Pittsburgh Penguins and the prospect of NHL expansion to 21 teams are eroding the Washington Capitals' committment to building a Stanley Cup champion through the amateur draft.

General Manager Max McNab saluted Pittsburgh couterpart Baz Bastien for his trading acumen the other day and said, "Pittsburgh showed what a team can do by trading away draft choices. When this season began, I thought if everything broke well we would have between 70 and 75 points and I thought with that total both L. A. and Pittsburgh were catchable.

"But Pittsburgh did a good job accumulating a solid club. They really made a leap forward with guys like (Randy) Carlyle, (George) Ferguson and (Rod) Schutt, guys who will be around a while. And (Ross) Lonsberry and (Orest) Kindrachuk give them leadership.

"If we wanted to take our three first-round draft choices for the next three years and trade them, we could have a hell of a hockey team, too."

McNab made it plain that he did not want to make a move. however, a fifth straight failure to achieve play-off status has made the club's predicatment a serious one. Season-ticket sales were down this year despite the 20 percent rebate plan and they figure to take another tumble.

Additionally, the merger dilutes the amateur draft to the extent that where a team previously could hope for twe solid picks a year, there is now only an outside chance of more than one. Ane the top clubs are that much more anxious to deal way surplus for the bottom teams' key no. 1 picks.

McNab knows the tempting offers from those lucky devilsin the top half dozen will be hard to resist. Yet he denies feeling any pressure to alter his long-range plans.

"We just recognize that if the proper deal comes, we will give it strong consideration," McNab said. "We will do what we have to do in the draft and then play our moves. If we go through this draft and trade our next three first-round choices, I'm sure we'd be highly competitive. Pittsburgh was in the same position and it worked for them."

There are other possibilities for capital improvement. Arne Stromberg, the club's European representative is in town and there is a possiblitity of another addition from overseas. PaulMackinnon, the young defenseman at Winnipeg, will be recovered as part of the merger. Tim Coulis, out with an injury all season, and Pierre Bouchard are expected to be 1979-80 assets.

Then, too, defenseman Rick Green's contract is up for renegotiation and he may be considered available for the proper offer. From the number of teams that have expressed interest in Green, a proper offer could provide the Capitals with substantial help.

McNab thinks the disparity among NHL clubs is so great that the bottom segment needs extra assistance but he is enough of a realist to know it will not be forthcoming.

"The league is broken down to two sixes and a five," McNab said. "I think the league should consider giving the bottom five clubs two choices before the others draft. I've suggested it, but it's never going to fly."

If it weren't for the need to make a giant leap forward to keep more loyal fans from straying, McNab would not consider abandonment of the buid-through-draft policy.

"To win the Stanley Cup, which is our goal, you have to accumulate as many class players as you can," McNab said. "You get them-the Clarkes, the Potvins, the Trottiers-through high picks in the draft.

"Look at the Islanders, how a couple of class players can explode things in a hurry. We want to look at 100 points, not to make 80 and stay there for five years. I wouldn't want somebody to take over my job and have the future mortgaged for him."

Bouchard was with the Capitals last night when they flew to Toronto. He may play in tonight's televised game (WDCA-TV-20 at 8 p.m) against the Maple Leafs, because NcNab thinks it would be easier for the big defenseman to make his Capital debut on the road. The only other road game left is at Montreal April 7 and the pressure there figures to be overwhelming . . . Left at home were injured Tom Rowe and Bob Sirois, and the absence of their goal-scoring punch will hurt Washington in its bid for a first-ever victory at Maple Leaf Gardens . . . Toronto is 1-5-1 in its last seven games, although goalie Mike Palmateer has been outstanding.