Brakes went kaput

[SubarcticPig]

I have a 1988 chevy k1500. i was driving it and the brakes went to the floor out of no where. i checked for any blown lines but found none, there wasn't any wet areas that i could find. so i bled the brakes starting from the farthest from the master cylinder and worked my way closer, very thoroughly, and created grade A pressure. But, as soon as i started the truck, boom right back to the floor with almost no pressure at all. i've heard bleed them again, bad master cylinder, bad booster, broken line. im positive they are bled as bleeded as bled could be bled, and still no wet spots or leaking to indicate blown or cracked lines

[Schurkey]

Originally Posted by SubarcticPig

I have a 1988 chevy k1500. i was driving it and the brakes went to the floor out of no where. i checked for any blown lines but found none, there wasn't any wet areas that i could find. so i bled the brakes starting from the farthest from the master cylinder and worked my way closer, very thoroughly, and created grade A pressure. But, as soon as i started the truck, boom right back to the floor with almost no pressure at all. i've heard bleed them again, bad master cylinder, bad booster, broken line. im positive they are bled as bleeded as bled could be bled, and still no wet spots or leaking to indicate blown or cracked lines

Brake pedal on the floor, NO external leaks?

HAS to be an internal leak if there's no air in the system. Since the brakes work sometimes, you can rule out air.

TWO possibilities; the more common and more likely is a defective master cylinder.

The less likely (but still VERY possible) is a defective Rear Wheel Anti-Lock (RWAL) hydraulic unit. The internal valves leak or seize, and the pedal goes low. The FRONT brakes would still function.

http://www.aa1car.com/library/abs_kelseyhayes_rwal.htm

[MT-2500]

Originally Posted by SubarcticPig

I have a 1988 chevy k1500. i was driving it and the brakes went to the floor out of no where. i checked for any blown lines but found none, there wasn't any wet areas that i could find. so i bled the brakes starting from the farthest from the master cylinder and worked my way closer, very thoroughly, and created grade A pressure. But, as soon as i started the truck, boom right back to the floor with almost no pressure at all. i've heard bleed them again, bad master cylinder, bad booster, broken line. im positive they are bled as bleeded as bled could be bled, and still no wet spots or leaking to indicate blown or cracked lines

When it first done it was the brake fluid level low?
On bleeding brakes a ABS cabable scanner is required to bleed ABS unit.

Engine offkey off pump up brakes 4 or 5 times and the release pedal and step on brakes.
Tell us how much pedal you have engine and key off?
Also does pedal seap own engine off and engine running with light and the hard pressure on pedal?

If fluid is leaving master cylinder look for and find fluid leak.
Your year truck has a lot of rusted brake line problems leaking fluid under high pressure engine running and hard push ing on brakes.
And sometime the line leaks or blows all together.

Jack it up on all 4 wheels and start engine and work brakes hard to you start feeing pedal dropping and look at all lines on all wheels and al lines from master cylinder to abs UNIT AND ALL LINES FROM abs UNIT TO ALL WHEELS including over fuel tank and left front whell well area to master cyinder wheel well back and over frame to abs unit.

Tell us what you find.

[Schurkey]

Originally Posted by MT-2500

On bleeding brakes a ABS cabable scanner is required to bleed ABS unit.

I don't think that's true for the POS Kelsey-Hayes RWAL unit. Needs nothing more than to have the bleeder valve opened, just like a wheel cylinder.

[MT-2500]

Originally Posted by Schurkey

I don't think that's true for the POS Kelsey-Hayes RWAL unit. Needs nothing more than to have the bleeder valve opened, just like a wheel cylinder.

You are right there.
My old eye saw 98 instead of the 1988 year.

But still sounds like a brake line leak.

[SubarcticPig]

thanks guys. i am replacing the mastercylinder as we speak. while i was underneath the truck i noticed that a couple of the brake lines have already been replaced, from the caliper to the split in the front and from the drum the the split in the rear, both on the drivers side. i got new lines and am going to replace the passenger side as well, as a preventative measure

[MT-2500]

Originally Posted by SubarcticPig

thanks guys. i am replacing the mastercylinder as we speak. while i was underneath the truck i noticed that a couple of the brake lines have already been replaced, from the caliper to the split in the front and from the drum the the split in the rear, both on the drivers side. i got new lines and am going to replace the passenger side as well, as a preventative measure

Good Luck and let us know how it goes.