Forces sales due to margin calls and lack of institutional buying interest continued to push the stock market lower yesterday as the Dow Jones industrial average dropped 6.75 points to close at 785.26.

The decline comes on top of a 15.08 point loss monday and puts the market at its lowest level since last April and decisively below the bottom it reached on Oct. 31 before President Carter's dollar intervention policy produced a brief but poweful rally.

The drop in the Dow was actually milder than the broad market's retreat. On the New York Stock Exchange, more than five times as many issues slumped as gained. The NYSE common stock index was down 0.47 point to 51.3.

Volume on the Big Board rose to 30.8 million shares from 21 million shares the day before.

Analysts said investors continued to worry about soaring interest rates and the spreading 11 percent bank prime rate, the souring of Mideast peace prospects and the gathering consensus of forecasters that a recession may not be too far off.

But the main reason give by stock watchers for the market's lackluster showqing was as simple as too many sellers and not enough buyers.

"We certainly have a lack of bids, and a lack of conviction on the buy side," commented Gary Helms, research director at Loeb Rhoades Hornblower. "We have a sober mood on the Street, and it's appropriate. But it's not fear and we're over the panic stage. Rather, investors are assessing the October debacle, the Phase II anti-inflaalation program, the new monetary austerity and what's going on in iran."

"The bids that are around and are available are somewhat sparse, and the buyers who are participating are scaling down," said jacques Therriod, a vice president with Smith Barney, Harris Upham.

Both Helms and Therriot, however, believe the market is probing for a bottom and may not drop much further in the weeks ahead. At the same time, analysts have little expectations ofany rebound from current levels.

The market yesterday was down more than 9 points earlier in the season, and recouped in the final hour as some selective shopping emerged.

Borders Inc. was the most active stock on the NYSE and dropped 5/8 to 26 1/8. Rorer Inc. was up 1 3/8 to 18 1/4, with company officials calling the rise, on heavy volume "a mystery."

Among gambling stocks, Ramanda Inns was up 3/8 to 7, Caesar's World was off 1/4 to 14 7/8, Harrah's dropped 1 to 16 7/8 and Del E. Webb dropped 3/4 to 11 5/8.

On the American Stock Exchange Resorts International A was off 7/8 to 25 25 1/6.

The American Stock Exchange index was down 2.25 points at 318.28. Some 105 issues rose whole 563 declined.

The NASDAQ composite index of over-the-counter stocks was off 2.12 points to 110.88.