Replacement master cylinder cap?

[Tarponhuntr]

Replaced Ed's front brakes tonight. When removing the master cylinder cap I discovered that the seal in it had turned to compete mush. In fact it came out a bit like chewing gum, some stuck inside , some stuck with the cap, with a small strand connecting the two.
Now, I have an air contaminated system and very mushy, yet functional brakes.
Is there a replacement cap with seal or even just a seal?
I'm going to have to rebuild my callipers anyway, so the entire system will be bled , but I don't want to do this until I can maintain a sealed system.
Thanks.

[shogun]

http://www.realoem.com/bmw/enUS/show...diagId=34_0494
Brake master cylinder/expansion tank

No. Description.......Supp......... Qty From......... Up To........Part Number Price
01 Brake master cylinder GIRLING 1 03/1988 34331156978 $527.00
02 Plug 2.......03/1988 34311163464 $6.95
03 Self-locking collar nut M8-8-ZNS 2 03/1988---04/1994 31331092887 $0.58 ENDED
03 Self-locking collar nut M8-8-ZNS3 2 03/1988 07119904295 $0.36
03 Self-locking collar nut M8-10 ZNS3 2 03/1988 07119905374
04 Brake master cylinder o-ring...1.......... 03/1988 34311159007 $3.15
05 Expansion tank GIRLING 1.............. 03/1988 34321160492 $85.93
06 Cap with warning switch......1............. 03/1988 34321162810 $30.33
07 Gasket..............................1............. 03/1988 34321157396 $4.52
08 Strainer tube.....................1............. 03/1988 34321120843 $6.35