My Husqvarna Hydrostatic transmission is not delivering the consistent drive power that it once had. It is stopping going up small inclines. I have replaced the drive belt with no improvement. I am planning on checking the torque spring to see if that helps but I was wondering if there was some other adjustments that I can perform to hopefully rectify the issue. My tractor is a YTH 150

Try this - remove all the transmission oil from the transaxle and put in a heavyer oil - 50 wt.
see if your transaxle has a drain plug - if not you will need to remove the tranaxle from the lawn tractor. I had to try a ratchet strap to the rear of the tractor and lift it up until the wheels were free. I removed the brake linkage spring, the fwd/rev linkage, and wheels. Undo the axle bolts and two screws ofn the cross support bracket on the tranaxle mount. pull the vent fill plug on top of the transaxle and turn the transaxle upside down to drain all the oil from it into a pan. Refill with 50 wt. to the top and remount the transaxle back in place. Next you will need to bleed the system as follows:
1. Have the tractor on jack stands with rear wheel off the floor.
2. Start engine on low speed.
3. repeat open and closeing the bypass valve while alternating the forward and reverse pedals.
4. when wheels start to turn increase the engine speed.
5. repeat quick forward or reverse with panic braking stops until transaxle gets full responce.
6. remove jack stands and have tractor on the wheels.
7. recheck the oil level again.

I did this to mine and it will improve your tractor ability to climb hills like new. 30 wt oil is too light (good for brake in) when tractor was new and refilling it with 50 wt will work like new or better.

If your Husqvarna hydrostatic transmission is like mine (2002 YTH1542XP), then it's sealed and you cannot change the oil. I'm currently investigation this issue as well because I've been having similar problems for years and I don't want to buy a whole new transmission.

A common problem on mowers with hydrostatic transmissions is the pulley on top of the transmission. Most have splines which wear out. Also make sure fan on top of transmission is turning. Could be overheating.

Husqvarna YTH 150 hydro transmission bypass will not disengage transmission

The Not Serviceable LIE

Hydro static transmissions on most lawn tractors are very similar. Many of them are made by the same manufacturer. 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 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 manufacturers fill 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 (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 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.

I have a 2013 Husqvarna 22V42. I recently started to have this same issue, so I did some research and discovered this thread. Indeed my model has a Tufftorq transmission, and indeed the manual says the oil is not user serviceable. Indeed, however, I was able to pop off the small round black plug on the top of the transmission, and I used a hand pump to remove the old oil and put in 50W. Fired it back up and.... bummer. That wasn't my problem. Here's what it was. The brake pedal has a linkage to the transmission, such that when the brake pedal is pressed in, the linkage pulls a level on the transmission to disengage it. When the brake is released, the lever springs back and re-engages the transmission. My problem was: after a few seasons of mowing my dry native grass areas, enough dirt/dust/dry clippings got trapped behind the lever that it was not fully springing back into position when the brakes were released! Cleaned out the dirt. Works better than ever.

I had this exact issue, but, the oil replacement wasn't the answer. I removed the transmission, pulled the plug on top, drained/replaced the oil, re-installed transmission - and same issue. My problem ended up being a very compacted build-up of grass/grime just in front of the lower dash area. This was preventing the forward/reverse pedal from going all the way down. It wasn't noticeable at all from a user perspective, as I suppose it had built up over time bit by bit. There was a TON of grass down there which isn't visible at all, and isn't easy to get to by hand. I ended up having to blast most of it out with a sprayer. It completely fixed the issue. Oh well... at least I know I have fresh oil in the trans, and things are all clean underneath. But, you might save yourself some trouble by first thoroughly checking this area out. Check not only the forward/reverse pedal side, but, the brake pedal side as well. If the brake doesn't fully disengage, it will affect the transmission pressure.

If it is losing power purge the transmission. Check the manual for instructions. We had this problem and purging it fixed the issue.

HOW MUCH DO YOU FILL IT?!