Might be that you need to bleed the tranny. Or perhaps the tranny has an internal problem. Following are some general notes. Go to the websites and review the tranny instructions.
The Huskvarna YTH150 uses Hydrogear transmission 319-0650, drive model 150071. One place to find parts and diagrams is here:http://www.m-and-d.com/Hydro-Gear_parts....
Hydrostatic Transmission Fluid is actually 10W-50 synthetic motor oil (like mobil).
Do not use hydraulic fluid or transmission fluid.
Hydro static transmissions on most lawn tractors are very similar. Most of them are made by one or two manufacturers. Tuff Torq makes a lot of them. They definitely make all the John Deere lawn tractor hydro drives. The lawn tractor manufacturers (like JD) often say they are sealed and not serviceable. That is flat out, a lie. All of the JD hydros, and most all others, are serviceable. A complete line of rebuild/replacement parts can be found at Tuff Torq. Info and instructions can be found there. It's quite likely they made your tranny too.
Additionally, I have found that the mower manufacturers fill them (or spec them) with motor oil, 10W-30W. In the service manual, and at the hydro manufacturer, you are told if your tractor is used in extreme or heavy duty situations it's best to drain the oil and replace it with 10W-50 synthetic motor oil (like mobil). Truth is the tranny should have this oil from the get go. It will give you more power and extend the life of the tranny. The mower manufacturers are all just being cheap. They figure why not, you're the one who has to pay for the early failure. Oh yeah, one other minor detail, the tranny manufacturer also recommends regular oil changes, like every 100 hours.
Virtually all of these trannys have a black plug on the top. You have to remove the tranny from the mower (not a difficult things to do, takes about 30 min). Pull that black plug and turn the tranny upside down for about 30 min to completely drain. Most hold about 2 quarts. Flip it back, right side up, and refill. Install plug and reinstall tranny. Note: under the plug there is often a magnet which collects metal shavings. Lift this magnet out while you have the plug out, clean it and replace it. It may fall out while the tranny is upside down. When you refill the fluid just keep filling till it comes up within a quarter inch of the plug. You may have to go through a bleed process after reinstalling the tranny. The tuff torq site explains the bleed process.