Hi Dan, So you have tried replacing the reverse solenoid?
The thread in the forum said that this was just a possible fix to the no reverse problem. If this didn't help then unfortunately It may be that your reverse drum is bad.
I don't see any valve body gaskets available in the factory BMW parts catalog. There may be other sources for a valve body gasket but I would not know who as we don't tear down the transmissions. BMW does not want their transmissions rebuilt at the dealership so they don't offer many internal parts.
Your best bet would be asking a transmission shop that specializes in rebuilding import transmissions in finding a valve body gasket. I'm sure they are available straight from the companies that make them for BMW.
Is there a reason your mechanic wants to open the valve body up?
I would not recommend spending more money if your mechanic is not sure it will fix the problem. A full transmission rebuild with 145,000 miles on it will definitely not be cheap.
If you check out http://noreverse.org/repair.html
,The typical cost to replace just the reverse drum and seals is around 1,600 dollars. This is assuming nothing else is found to be bad.
If you could find a used transmission at a salvage yard and have it installed, this is always another option.
Edited by Lee on 1/12/2010 at 6:02 AM EST