aggravating problem-tractor won't stop up hill only

A few weekends ago I broke the drivebelt on our 42" 18.5 HP tractor. Its a gear driven unit. Model 917.275631. Anyway I replaced the drivebelt, mower belt, blades and both mandrels. The mandrels were worn down where you attach the blade, causing a bit of a wobble. Since then I have a very interesting problem. While mowing uphill the tractor won't stop. I depress the brake fully and it just keeps going. If I'm on level ground or downhill, no problem. I have noticed there is more play than I remember in the brake pedal then what I remember before and it does take a bit longer to stop on level and downhill runs. I'm convinced this is some type of adjustment. I haven't been able to find anything in the manual. Any help would be appreciated.

Thanks

Bob