I have a 98 GMC Sonoma/Blaz 4.3 4 by 4 Pick Up.I replaced the vaccume hoses one by one(4 by 4 not working). After replacing everything works OK.However the vaccume reservour fills with ATF and drained the trans more than a liter of ATF! I disconnected the main vaccume line at transfer case and plugged both ends.This solved the leak but WHY is the vaccume switch on the Transfer case (3 hoses)have ATF drawn thru hoses ? Also the coupler for the hoses on the top rear of transfer the rubber connector spins arround where vaccume hoses are connected?
What can be done? Much appriciated any suggestions.

Thanks 48chev

 

It sounds like you may also have a faulty transfer case input seal. Pull the fill plug on the back of the transfer case. If ATF pours out, the transfer case needs to be dropped and the input seal changed.

You will want to check the HVAC system for ATF intrusion. It can gum up the vacuum actuators in the dash.