I have a 94 gmc sierra 1/2 ton.The m/c went out and and I put a new one on.The truck has abs on the rear only.But I don't have problems with them just the front.I bench bleed the m/c,bleed the brakes.Starting at the right rear first.No air until I drive it for 2 or 3 miles.And then brakes get soft and there's air again in m/c.I have got 3 new m/c's 2 gallons of brake fluid.Theres no leaks or loss of fluid.Any help would be great.The fluid has millons of air bubbles.almost milk color until it clears up.thanks

 

It almost sounds like you are boiling your brake fluid, check calipers to make sure they are functioning properly.

jvp

 

I have checked the calipers and they seem to be working fine the fluid isn't hot.