I have a husqvarna rider 155, the transmission loses power after I have ben mowing for an hour

Try this. Remove the solenoid from the mower, this makes access to test it easier. Apply 12 volts to the two SMALL terminals on the solenoid. You should get a clear "Click" as you turn the power ON and again as you turn it OFF.
With the power ON use a multimeter set to Ohms or Resistance and test across the two LARGE terminals. you should get no resistance. When you turn OFF the power it should change to a resistance reading.

There a breather hole or plug on top of tran.

Husqvarna 155? Sounds like the fuel shutoff solenoid on the bottom of the carburetor is toast! I'd say replace it BUT in the long run, you're better off at replacing the whole carb assy.

check the drive belt, maybe slipping

Can you push the mower by hand?

Well if new I would take it back and if used I would be looking for another hydrostatic transmission.
These can be found on ebay, and there are some lawnmower recyling yards out there, you have to look for them. The price of a new transmission far exceeds the price of a used mower and is almost the price of a new mower.

Try submitting your question to the correct category.

Not an expert but have a rider 155. After looking at diagram on partstree.com, it does not appear to have a filter. Just a magnetic drain plug. This is the only site I have found for info and parts.

Under the mower below the seat of the rider, at the rear of the tractor, look up and you will find the transmission there. There is a belt that runs on the pulley under there, and this belt can slip off when it gets worn or stretched. Sort of a tough spot to get to but, that is most probably cause.

Let me know,
Chip

we have tractors and, yes any of those drain bolts will work (for best results do both) then after its drained, put the drain bolts back in and top up the level