Spongy Brakes, ABS light 2006 Itasca Sunova 26P

[GPanek]

I have a 2006 Itasca Sunova 26P Class A, Workhorse chassis. After about an hour on the freeway I turned off onto a surface street and found I had to pump the brakes. Soft and spongy. They would stop the MH but only after pumping. Very scary. I stopped and checked that I had plenty of brake fluid in both reservoirs. When I arrived at our destination the ABS light came on. So I took out an ABS fuse and replaced it, sort of a reboot. Let the vehicle sit for four days while we camped. And by the time I left the brakes were solid again and shortly after driving a mile the ABS light went out. No leaks anywhere. The only thing I could think of was really old, contaminated brake fluid. So I replaced all the fluid when I got home. Had to run about 4 qts thru. ABS lite came on at first but went out after driving a couple hundred yards. Had the brake pads checked recently and all had plenty of depth. Any ideas on what it might have been if not the old fluid? Thanks

[Ray,IN]

Workhorse chassis; does it have the hydro-boost/hydro-vac brake booster Chevy used? When mine went bad on my Chevy 1T truck that was the way my brakes acted.
When you replaced the brake fluid, the Hydro-boost requires special equipment or the shade-tree method:

Hydro-boost braking system bleeding instructions..

[GPanek]

I don't know if I have hydro-boost or not. I do know, there was no steering gear or pump installed recently. I certainly can do the procedure in the link you provided to see if that helps. They refer to fluid level in the pump. Is that referring to power steering fluid or brake fluid? Anyway, thanks for the tip, I will give it a try.

[Ray,IN]

The hydro-boost uses the same power-steering fluid as the power steering.
Here is another pdf instruction on bleeding/purging air from the Hydro-boost system:

https://my.cardone.com/techdocs/pt%2052-0001.pdf
This a GM hydro-boost unit and its location:

[Adven2011w24]

Sep 21, 2017 — Have you had the Bosch brake recall (50901-C) performed? Workhorse will replace all 4 calipers at no charge. This recall covers a symptoms you seem to have.