Mystery leak - VP DP-SM drive help?

From the wealth of experience I see from contributors to this board, I am hoping someone has some insight that could help locate a mystery leak in a Volvo Penta DP-SM duoprop drive. In summary the drive holds pressure and vacuum yet gets water in the oil after a few hours running. On start up you can see the occasional oil spot on the water behind the drive. It has been back to the shop a couple of times and they are mystified as well. I was hoping someone has some experience of this type of leak and could point me in the right direction?
Here is a history:
Propshaft and shift seals replaced November 2008
Drive checked during haulout June 2009 - no water in oil, slight top up needed.
Oil on water noticed behind drive on startup fall 2009 - haul out, drive oil level low, no water in oil. Pressure checked OK.
April 2010 - noticed oil spot on water while cleaning off props, engine not running.
May 2010 - oil on water behind drive on startup again, haul out, water in oil. Cleaned propshaft seals, vacuum and pressure test OK. Back in water.
I fired it up the other day, still leaking.

Everything points to the propshaft seals, yet the drive holds vacuum and pressure. Any ideas?

Thanks

Re: Mystery leak - VP DP-SM drive help?

When they do the pressure and vacuum checks, do they drain the oil first? If not, then they never will find the leak.

Re: Mystery leak - VP DP-SM drive help?

Thanks Don.
By all accounts the drive was drained before testing, but there is still going to be some oil left around the seals. They did report some grooving in the area of the seals but commented that it did not seem bad enough to cause a leak. They also vacuum and pressure tested it while spinning the propshafts, no signs of leaking.

They are a reliable shop and generally seem to have expertise in what they are doing. One suggestion is to change the propshafts which is a major expense as it includes the gear set.

Does pressure and vacuum testing always show where a leak is occurring? Have you come across anything like this before?

Re: Mystery leak - VP DP-SM drive help?

Yes sometimes they can pass the tests but still get milky lube.
Have seen this before but more on bravo 3 than VP.
Sometimes you need to stand on the inner prop shaft while doing the low (4-6psi) and the high (14-16psi) pressure and vacuum tests.
Also should check prop shaft run-out and play (Volvo makes a very easy to use tool for this).
If you do pull the bearing carrier and tube gear, take a close look at the lips of the seals. If the gear lube has washed the grease out this sometimes points out which seal is leaking.
Good Luck Muc

Re: Mystery leak - VP DP-SM drive help?

Thanks for the suggestions, I will ask them about that.