what is wrong with this
 

my wife uses our 99 blazer for the mail route and now shes leaking brake fluid.the lines come out of master cylinder and go into the box next to it.im assuming this is for abs?well now i see a little bit of brake fluid is leaking out the little hole on the end of it.i had her pump the brakes and a little fluid came out of it.what is the proper name for this and is that bad or my master cylinder?ive searched for this problem but came up blank.
thanks for any help its greatly appreciated!

That would be the ABS module and it should not be leaking any fluid.

The lines from the master cylinder first go to the proportioning valve which can be serviced separately from the ABS unit. If brake fluid is leaking from the rubber cap, the valve is leaking and needs to be replaced.

Attachment 33808

yes the part circled in red looks like that has the "bleeder looking part where its coming from.i could find a diagram last nite thanks alot now i know what to order.
now i gotta trace a fuel smell coming thru vents.she said it just started it but i cant get it to smell now?

there is no rubber capon mine its just an open hole

no its part number 4.would that be the abs modulator valve?

Part #4 is the ABS modulator and should not leak at all. It's usually referred to as the EBCM, (Electronic Brake Control Module). If it leaks, replace it. There are places online that you send them yours and they send you a remanufactured unit. Or you can try a boneyard.

yeah im tryin a boneyard first.they are pretty pricey.ebay has em for 250 though.everywhere else 500 on up

ADAM 10 my girlfriend also drives a mail route she also uses a 99 blazer for said route and another coincidence im haveing the same problem with that same part the combination/proportioning valve is leaking from that bleeder end were u able to find and replace the part from a junkyard?