E-450 Transmission brake constant leak

[Nomad Brad]

Hello Ford Family! I have an ongoing issue that no mechanic has been able to resolve, you guys are my last hope.

I have a 2007 Ford E450 box truck 135,000 miles ( EX uhaul ) A mechanic misdiagnosed a clunking noise as a failed transmission mounted parking brake ( connects between the transmission output shaft & driveline ). I Installed a replacement unit from Amazon and tossed out the OEM unit. Turns out the original unit was not the source of the problem.

But now a new problem has been created. The replacement unit from Amazon started leaking fluid from the parking brake housing (not where it attaches to the transmission )soon after I replaced it. I took it to a trusted mechanic who said it was likely a poor-quality replacement part so he ordered a new parking brake assembly from his supplier. That unit started leaking soon after installation. Then he rebuilt that unit with better-quality bearings and seals. same problem, leaking soon after installation. I thought maybe I would have better luck with an OEM unit. I found a guy who sells rebuilt OEM units. This guy said he has been rebuilding them for years and never had one of his rebuilt units leak.I Installed it and same problem, leaking a few miles into the first test drive.

I have now installed 4 "new or rebuilt" transmission parking brake assemblies. All of them start leaking ATF within a few miles of driving. With every unit, the leak progressively gets worse. I created a dipstick to measure the fluid level from the fill hole. At first, I can go a couple hundred miles until I have to refill it. Eventually,  I can't drive more than 20 miles without all the fluid leaking out. It Leaks at a faster rate when I'm driving at freeway speeds versus around town.

I am at a loss here. I can't help but think there MUST be some secondary issue stressing the assembly and causing it to leak. 

The strange thing is that it never leaked before I replaced the original unit that was misdiagnosed. Also the u joints and carrier bearing have been replaced. There is no noticeable vibration or noise coming from the drive line or transmission. Shifting is smooth and strong.  Any suggestions on what I can do? every mechanic I talked to has no advice other than just replace the parking brake assembly. 

any help is appreciated, This is my work van and I have a ton of money into it, I cant afford to just scrap it and buy another, but I don't know what else to do. 

Thank you,

Nomad Brad