DEAR BOB: Eight years ago we sold our home and took back the mortgage for the buyer. Everything went fine until a year ago when the couple got a divorce. The wife got the house and has been having a hard time keeping up the payments. She is now four months behind. How long should we wait before filing foreclosure papers? She keeps promising to mail us the money but she never does. Her checks have bounced twice. Mr. W. W., Chevy Chase.

DEAR MR. W. w.: You've waited too long already. Never let a borrower get behind on mortgage payments. If they can't pay you today, they probably won't be able to pay you tomorrow. By promptly filing the foreclosure papers, you'll show the borrower you mean business. Most mortgage defaults get cured before actual foreclosure becomes necessary. The sooner you file, the sooner you'll get your money.

DEAR BOB: Some time ago you explained that a person selling his home who buys a more expensive replacement must defer paying his profit tax. Must all the cash from the sale be reinvested in the replacement? Jose E., Arlington.

DEAR JOSE: No. The tax law, Internal Revenue Code section 1034, says that when you sell your principal residence and buy a more expensive replacement, within 18 months before or after the sale, you must defer paying tax on your profit until you sell the replacement without buying another.

The law says absolutely nothing about how much of the sale proceeds you must invest in the replacement. Smart home sellers can buy their replacement home with a maximum mortgage, such as a 100 percent GI mortgage, and spend their tax-free cash as they please. For details, see your tax advisor.

DEAR BOB: I have two sons. One is a bum, the other is terrific. I don't want the bum son to receive any of my properties when I die. Can I give my good son a quit-claim deed to my properties now with verbal instructions to record the deeds after I die? Thomas C., Alexandria.

DEAR THOMAS: You can do that, but it won't be legally effective. The reason is that to be valid a deed must be unconditionally delivered during the grantor's lifetime. Delivery, with the condition the deed not be recorded until after you die, would be invalid. Talk to your attorney. He or she can suggest better methods of making sure the right son gets your realty.

DEAR BOB: I just bought a rental duplex. When I went to my tax man to set up the books for it, he said that if I use accelerated depreciation when I sell the duplex, my depreciation will be "recaptured" and taxed. Is this true? If so, what's the advantage of accelerated depreciation? Sue M., Washington.

DEAR SUE: Don't be scared by depreciation recapture. It simply means that when you sell a depreciable building on which you've been taking accelerated depreciation, the difference between accelerated and straight line depreciation is "recaptured" and taxed as ordinary income.

For example, suppose you deducted $5,000 accelerated depreciation during your ownership of the duplex. But if you had used the straight line depreciation method, suppose you would only have deducted $3,000 depreciation. The $2,000 difference is "recaptured" and taxed as ordinary income at the time of the sale. In the meantime, however, you had the tax savings benefits.

DEAR BOB: About two months ago we hired a professional appraiser to appraise our home's value (we are getting ready to sell it). I thought his appraisal was low so we asked our banker to recommend another appraiser. The second appraiser's appraisal was about $3,750 higher than the first one.Both claim to be "professional appraisers" but they belong to different appraisal societies. Why the difference in appraisals? Beth M., Rockville.

DEAR BETH: Property appraisal is not an exact science. Each property is unique. For this reason, two expert appraisals will differ as to a specific property's exact market value.

The important thing to learn about an appraisal is how the appraiser arrived at his market value evaluation. Did he use recent sales prices of comparable neighborhood homes? Which ones? Did he use the "replacement cost" method as a double-check? If income property is involved, the capitalization method of valuation may be appropriate.

When selecting an appraiser, check his or her credentials. If he or she has several years of residential appraisal experience, belongs to one of the major appraisal societies, and is familiar with your area, the appraisal should be highly accurate.

DEAR BOB: Where can I write to get details on FHA and VA home loans? Oscar M., Leesburg, Va.

DEAR OSCAR: Don't write. Just visit the nearest office of a FHA-VA-approved bank, savings association or mortgage broker. The loan officer can give you accurate information about the types of FHA and VA loans you can get.

DEAR BOB: I'd like to get started buying investment property. Is it a good idea to first form a corporation to take title? Ron W., Fairfax.

DEAR RON: No. Disadvantages of corporate realty ownership include double taxation of profits (once to the corporation and again as dividends to the stockholders) and no pass-through to stockholders of paper tax losses. Your tax advisor can explain further the disadvantages of corporate realty ownership.

DEAR BOB: Like many other young couples, we would like to buy a house or condominium. But with high prices and high interest rates, we're very discouraged, as we've only managed to save about $3,500 in two years of marriage. But we earn a total of $19,000 annually. Any ideas? Joseph M., Washington, D.C.

DEAR JOSEPH: You can probably afford to buy a house or condo priced at 2 1/2 to three times your total annual income -- $57,000, maximum.

(1) If you or your wife qualify, you can get a VA mortgage for 100 percent of the purchase price. (2) FHA home loans are available up to $60,000 for a cash down payment of only 3 percent of the first $25,000 plus 4 percent of the balance. (3) PMI (private mortgage insurance) mortgages are available from more than 22,000 banks and savings associations for 90 and 95 percent of the home's market value.

(4) But your best finance source is the home's seller. Your realty agent can probably find you a home where the seller, perhaps a retired person, will carry back a first or second mortgage for you. This technique will avoid having to get a new first mortgage; but be sure the old mortgage doesn't have an enforceable "due on sale clause."

(5) A lease with option is a technique I've used to buy and sell homes many times. It's good for both buyer and seller. For example, you could lease a home for a year, with an option to buy it in 12 months. The owner may give full credit (as I do) for rent paid toward the option purchase price. A knowledgeable realty agent can help you with this low cash method.

DEAR BOB: I own a small apartment house. What is the criteria for determining the difference between a tax-deductible repair and a capital improvement that has to be depreciated? Carlton M., Washington.

DEAR CARLTON: A repair merely preserves your property, whereas a capital improvement enhances or extends its useful life. For example, if you patch a leaky roof, that's a repair, but if you replace the roof, that's a capital improvement.

Most apartment owners prefer expenditures to be classified as repairs since they can deduct the expense in the year of payment. But capital improvements must be depreciated over their useful life. Ask your tax advisor for further guidance.

DEAR BOB: Do you think we should talk to more than one realty broker before we list our home for sale? We only know one agent in town, the agent who sold us our home several years ago. Frankly, we weren't too pleased with her but we hear she is one of the top agents here. Mary T., Lanham.

DEAR MARY: Yes, you should talk to at least three active agents before listing your home. You'll find there is a big difference in services offered among different firms. Before listing, ask each agent for references from their previous seller clients. After checking the references, list with the agent you feel will do the best job.

The new Bruss report, "How to Buy a Home for the Lowest Price and on the Best Terms," is available for 25 cents plus a self-addressed, stamped envelope sent to Robert J. Bruss, P.O. 6710, San Francisco, Calif. 94101.