85 Vanagon brake fluid leak

I have an 82 Vanagon that came to me leaking brake fluid from behind the dash. My problem seems to be that the brake fluid reservoir is simply pressed into the top of the master cylinder with two rubber grommets providing the seal. If the grommets get soft with age or improper brake fluid (I think newer DOT 4 and 5 or silicon brake fluids may deteriorate the grommets) the contents of the brake fluid reservoir leak out past the grommets until the reservoir is nearly empty. The brakes still work as enough fluid remains to keep air from entering the brake circuits. I replaced my grommets which seemed to solve the problem for a time but the leak has returned. I haven't gone back in to check the issue as I've been working on other issues. The van is roadworthy again now in all other respects so I'll be checking this issuemoutmand will report back what I find.

Scott