diaphram under gear oil reservoir cap

[edwardh1]

What does the diaphram that is in the cap on the sterndrive gear oil reservoir do? 4 cyl 1999 alpha one (I think alpha one)
Background- we thought/think we had have a gear oil foot leak- no visible sign except between this season and last season there must have been a leak as the level was down about 1/2 inch.
-I added fluid. Marked the level. Call it mark NUMBER ONE
-used it one time for 1 hour fluid dropped about 1/8 inch, put on mark 2
- used it 2 hours, it seemed to drop another 1/16.
when I opened the cap (boat on trailer) it went "psssit" , and the level jumped back up to mark number one.

It seems the cap was pressurizing the system to the the extent of pusing the level down probably expanding the gear oil level hose? - when you opened the cap the pressure came off and the gear oil level rose.
Do I have a defective cap? -

 

[Don S]

Re: diaphram under gear oil reservoir cap

Do you have a defective valve, NO, it's working as it should. It acts as a check valve. When the gear lube heats up, it expands. The reservoir builds up a little pressure to force the gear lube back into the drive.
If you have a leak in the drive, loose gear lube, and the level drops, the vent releases and allows the gear lube to drop without creating a vacuum. Thus allowing gear lube into the drive so it stays full.