After tax-season kept prices strong in the lanes earlier this spring, May depreciation rates showed signs of the seasonal “summer slowdown.”

Average prices fell by 0.4 percent last month when compared to April rates, according to Black Book data. On top of normal seasonal changes, a softening in the lanes is also being spurred by dealers preparing for fall new models, Black Book said, which is lessening demand at the auction.

Specifically, domestic car prices fell by 0.5 percent; import cars saw a drop of 0.9 percent; domestic truck prices increased by 0.5 percent; and import trucks dropped by 0.4 percent.

"As we’ve seen firsthand from the dealer comments collected by our survey personnel at wholesale auctions, the summer season is kicking in and there is less activity around filling the car lots right now," said Ricky Beggs, editorial director at Black Book. "Many dealers will begin to prepare their inventory strategies for the incoming new model year in a few months, depicting higher depreciation between now and the end of the year."

But not every segment is succumbing to seasonal trends.

Full-size passenger vans led all segments with the strongest monthly retention, with rates growing by 2.5 percent.

This segment was followed by full-size cargo vans, with a lift of 1.7 percent.

An improving economy and high construction rates are pushing strong retention in the van segments.

"There continues to be healthy demand for cargo vans because of the construction activity and the improving economy. We anticipate this will continue into the near future. Passenger vans are holding their retention right now because this is a category that is in sync as far as supply and demand," Beggs told Auto Remarketing.

For the car segments, the smaller rides took the lead in May for price retention, perhaps a result of elevated gas prices.

Compact cars saw prices rise by 0.8 percent in May.

Beggs explained the trend is mostly caused by seasonality: "The strong compact car prices are a result of the strong spring selling season, where buyers are very focused on this price point for a used vehicle. We anticipate that this segment will see rising depreciation as the year moves forward."

The average compact car price at auction at the end of May was $9,383, which represents a 6.8-percent drop from year-ago levels ($10,668).

On the other hand, the luxury-level cars led the way for depreciation, falling by 1.5 percent in May.

The average segment price at the end of May was $20,190, a 15.5 percent slide from year-ago levels ($23,882).

"Even though these change levels are greater than the remainder of the market, they fit within normal pre-recession depreciation levels," the Black Book analysis pointed out.

Luxury vehicles weren’t faring much better in the truck segements; the luxury SUVs saw the biggest price drop for the trucks, with a price decline of 1 percent.

Beggs explained the luxury segments are undergoing natural depreciation patterns and are only on the bottom end of the chart because the majority of the other segments are still outperforming seasonal patterns.

"The tax season  is not supportive of the luxury market so that has led to the seemingly greater depreciation levels," he added.

The story is different for pickup tucks, as many continue to see price strength in the lanes.

In fact, four of the top eight best-performing segments of the month in May were the full-size pickups ( up 0.6 percent), midsize pickups (0.5 percent), full-size SUVs ( up 0.2 percent) and the mid-size SUVs ( up 0.1 percent).

As for what may be in store for the rest of the year, Beggs said, "We expect typical seasonality through the remainder of the year and we will settle in at roughly 13.5 percent depreciation across all segments by the end of 2014, slightly elevated from last year's level of 12.6 percent."