Hawk 250 rear brake master cylinder replacement

[ChrisWNY]

Does anyone know off-hand if this aftermarket rear master cylinder will fit the Hawk 250?

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

The problem I'm having is that after a few hours of riding the rear brake goes out, provides no stopping power. After flushing with new DOT 4, works great, but that's only temporary. I've checked for leaks all over - no brake fluid getting onto the rotor and the reservoir is always full when I bleed the rear brake. The pedal pumps the fluid out just fine when I flush it out through the bleeder valve. My best guess at this point is that a minute amount of air is getting into the rear brake through the master cylinder somehow. Brakes do not drag at all after bleeding, and as I said the rear brake does work quite well for a fixed amount of time. The part on Amazon looks to be an exact match and would arrive quickly, vs. sifting through eBay (most of the universal brake cylinders are sold out of China which would take months to arrive).

The Amazon rear brake cylinder looks pretty much exact to the OEM part on the Hawk based on the photo of my own (below)...