Not flat tires but flattened tires. I'd 6'5" and about 360lbs. Cycling is not not new to me, but the weight is. I have been about 255 most of my adult life. I'm looking to commute to work for some exercise but my tires flatten out when I sit on the bike. I have an older Diamondback Sorrento ridged frame MTB that I've had for 15 years or so. 26"x2" wheels. 65lbs max pressure rating on Kenda hybrid tires. Any suggestions on how to solve this?