1996 chevy silverado brake question

I have a brake question. I have a 1996 chevy Silverado. I have had it for about 4-5 years. I bought it from my dad. It has 315,000 miles on it. It has always had a soft spongy pedal. Brakes seem to work, but
like I said the pedal sinks when holding it.
This weekend was a nightmare. I replaced one side of my back drum brakes. I will have to finish the other side this weekend as I ran out of time. I noticed when I was trying to force on the pads in the back, I developed some liquid at the master cylinder boot.
This weekend is a long one so I plan on doing this right. Replacing the other set of rear pads. I guess I need to replace that wheel cylinder now too. (Will I hurt anything by driving the truck this week with one set of new pads on one set of old pads?) For the record, the pads had a lot of life left on them but there were some superficial cracks so I attempted to replace them.
Also, for the last year my antilock brake light has been on.
Anyway, Part of me is afraid to replace the master cylinder and re bleed everything. I have heard of people doing this and somehow getting air on the anti lock module? Have you heard of this?
I appreciate any help and advice! I just want to get this done and be finished with it. This summer by the way I had to replace my vacuum booster. It went out. I have no fluid leakage that I can see by the way.
I appreciate any help!
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If the rears are drum, make sure the pads are shaped to the drum and adjusted with a little drag. Most spongy brakes are caused by incorrectly adjusted rear drums. Took me some time to get this worked out, but works like a champ when setup right. As for air in the system it can be bleed out. I usually run 2 -3 quarts thru the system. if the ABS motor is still running, you can force it to cycle with the proper tool. 95's were easier, there was a thread about this not too long ago.
Cheers
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I dont have a scan tool. Can I still replace the master cylinder and bleed the system?
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