Brakes become spongy

[lucusbumanlag]

I have a 1972 c10, small block 350, power brakes. It does perform pretty good not so bad while driving, the truck does stop. But when im at a stop light, sitting there, the brake pedal slowly depresses further and requires more pressure to keep the truck from moving. Could it just be that theres air in the system? Any thoughts?

[motorcritter]

You've got a bad master cylinder- internal leak, from one section to the other. I would inspect the whole system, assure the integrity of it (no leaks or nasty hoses), and put a new M/C on it. Bench bleed it, and make sure you don't bottom out the master cylinder piston when bleeding the brake system itself. I always put a foot below the pedal or a block of wood or brick to prevent full movement. Air in the system is not so subtle- brakes 'pump up' until they work, and still feel 'squishy'. Bad master cylinder will give a decent pedal on initial application, then sink, it will pump up, too, just not the same sequence as air in the lines/calipers/wheel cylinders. Good opportunity to flush out all the old fluid, get a good hard look at everything regarding brakes, and clean everything up.
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[MagmaJct]

Originally Posted by motorcritter

You've got a bad master cylinder- internal leak, from one section to the other. I would inspect the whole system, assure the integrity of it (no leaks or nasty hoses), and put a new M/C on it. Bench bleed it, and make sure you don't bottom out the master cylinder piston when bleeding the brake system itself. I always put a foot below the pedal or a block of wood or brick to prevent full movement. Air in the system is not so subtle- brakes 'pump up' until they work, and still feel 'squishy'. Bad master cylinder will give a decent pedal on initial application, then sink, it will pump up, too, just not the same sequence as air in the lines/calipers/wheel cylinders. Good opportunity to flush out all the old fluid, get a good hard look at everything regarding brakes, and clean everything up.
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I'll second that! I just rebuilt mine a few months ago for the exact same symptom. I had to hold the pedal firm to keep from moving forward. As you'd imagine, my leg grew tired of that quick!

When I disassembled the MC, and compared the old and new seals, I could see how much the old seals were worn. They have a cup shape to them. The wall of the outside of the "cup" was very thin.

I was not impressed by the quality replacement MCs out there, that's why I opted to rebuild mine.

Regards,
Mike