I use a flexifit 432 mask. I have always thought the medium was a bit too small. Is the only difference between the medium and large the seal? Is the plastic part the same on all sizes. I see on ebay I can get medium and large seal for $20.00 (Aftermarket) But I didn't know if the large seal would f...

My flexi fit 432 did the same thing. If your cpap machine has c-flex settings you need to turn it to a higher setting. Mine was set on one. I changed the flex setting to three and the problem was gone. The c-flex setting stops the air flow when you exhale.