[RockyMtnRay]

Never heard of valve damage due to use of engine braking...but suppose it would be possible if you let the engine grossly exceed the redline RPM. And I've never heard of any damage due to "pressure buildup" either...modern electronically controlled automatic transmissions manage their internal hydraulic pressures quite well. There's a whole lot of just plain bad information floating around that seems to get passed down through the years.

As Bill explains above, using and replacing wheel brakes is less expensive than replacing engine/transmission components...if...mighty big if...you can avoid brake overheating (and possible loss of brakes due to fade) when only using the wheel brakes to control speed while descending a long, steep mountain grade with a trailer in tow.

The one thing that engine braking does do...and that's hardly ever mentioned...is put a lot of loading on the thrust washers & spacers that keep all the helically cut gearsets aligned when torque is coming from the driveshaft instead of the engine. When you put torque on helical gears (which are extensively used in automatic transmission planetary gearsets), they tend to move laterally and thrust washers prevent that. Automatic transmissions have heavier duty thrust washers for front->rear torque passage (power on) than for rear->front torque passage (power off engine braking). Therefore, to avoid undue wear on these lighter duty thrust washers, it's a good idea to limit use of engine braking to those circumstances where overheating of the wheel brakes is very likely if engine braking is not used.

On nearly every trip when towing, I have to descend multiple 6 to 8 percent mountain grades of anywhere up to 10 miles in length. I therefore always use engine braking (3rd or even 2nd gear) as the primary control of my descent speed on those grades. I sporadically use the wheel brakes as needed to supplement the engine braking for speed control and for emergency braking as needed. Because the wheel brakes are not being steadily used, they remain cool and have their full effectiveness at all times. With nearly 7,000 miles of mountain towing on my truck I've yet to see any indication that using engine braking is causing any problems.

I've watched folks towing trailers (including professional truck drivers) try to descend Colorado's mountain grades using only the wheel brakes (almost invariably the rig has license plates from a flat/lowland state). It's downright scary to watch the smoke billowing from the wheels, especially considering that brakes that are overheated that badly are on the verge of complete failure. Nearly all the long descents in Colorado have "runaway truck ramps" and these get used on average 3 times a week!

As for any deleterious effect of using engine braking on automatic transmission fluid temperature, there isn't one that I've seen. Nearly all (if not all) automatic transmissions are cooled wholly (or at least in part) by a cooler in the bottom of the radiator...even if the vehicle has an external air/oil cooler. On a long descent with the throttle closed, the coolant in the bottom of the radiator drops quite low (to maybe only 20 degrees over ambient) due to abundant airflow in combination with little energy being produced. The net result is I typically see my transmission temperatures steadily drop to nearly as low as non-towing temperatures during a long descent using engine braking...even if at the top of the descent the transmission temperature was well over normal because of heating during the climb on the other side. Granted, with the torque converter unlocked while using engine braking in 2nd gear, there has to be some heat being generated in the transmission due to fluid slippage. However, my empirical evidence is that whatever heat is being generated, it's being easily handled by the transmission's cooling device(s).

Every make (and sometimes even model) of vehicle has different transmission designs and usage guidance in the owner's manual. Therefore the owner's manual should be guide. My owner's manual states to use 3rd gear (OD off) for better "engine braking" while towing. And that's exactly what I'm doing...but only for speed control on long, steep descents. For routine level ground deceleration, I use the wheel brakes because they're designed for that and a routine stop shouldn't overheat the brakes.