Brake pedal soft only with engine running

[duanes7]

Bought the 97 K1500 in SC and drove it here to middle GA. Brakes died just before parking it at my house. It sat for about 5 months until I could begin to work on it. The brake system was dry when I started the work and unfortunately the pedal had been pumped that way. One of the front hard lines and the rear hard line both were rotted through and I replaced them (see http://www.gmt400.com/forum/showthre...iy-brake-lines). Then, when I went to bleed them one of the wheel cylinder bleeder screws was rotted away. Replaced the wheel cylinders and the calipers. Also put in a new master cylinder for good measure. Bled the brakes using a vacuum system and then again with speedbleeders.

Ok after the novel...

I have great pedal pressure until I start the motor then the pedal goes to mush. The brake warning light is on and the pedal is still mush even with the power plug undone on the abs module. I have good vacuum at the booster.
Is this an ABS bleed problem? Been all over the net in the last few hours and it looks like the only way to bleed the ABS module, short of buying a $500ish tech 2 scan tool, is to take it to go to the dealer.

Any other ideas?

 

[Swims350]

what year truck?

make sure the flex lines are changed also, my rear gave me very similar problems, change them they are cheap and wear out over time being rubber.

if you got the extra coin get braided ones they don't give as much under pressure.

2nd make absolutely sure the rear shoes(if drums) are good shape and completely adjusted out to be almost touching or barely touching the drums when installed, this will definately cause soft pedal, mine did.

 

[95Escahoe]

X2 on the braided lines run the Russell ones on my truck they firm it up a lot

Sent from my HTC One X using Tapatalk 2

 

[Swims350]

just to clear it up a bit I could get a nice firm pedal on my 88 with the engine off very little travel, maybe an inch if that but once running it would hit the floor just about and with pressure it could and would set the brake light.

I had everything new, removed the RWAL abs crap, nw MC, new calipers, wheel cylinders, brake shoes and pads etc. I even put new ront flex hoses on and bled the system over and over numerous ways with no luck,

I changed the rear flex line, tried it and was a tad better, adjusted my shoes out in back and now it's very good.

 

[duanes7]

Just got back from the stealership. Took it there so they could bleed the abs. I had three codes c0066, c0073, and c0086. Anyone know what therse are? The technician didn't even try to run a functional check, just did two squirts of fluid each from all four corners and charged me an hour of labor. Didn't earn their five starts today...

The pedal isn't just soft like if I had a line flexing. It hits the floor and will barely stop the truck on level ground at 10 mph.

I suspect the abs has been worked on before as the one in the truck right now has writing on it like it came from a junk yard.

Brakes worked good from SC where i bought it to here in middle GA. Gave me one hiccup just before I parked it.

 

[TylerZ281500]

are you sure your not leaking any brake fluid? also check your vaccuum lines on the intake to brake booster.

 

[sewlow]

Those exact numbers didn't come up, but from the ones in proximity here, it appears to be an ABS prob.

http://www.obd-codes.com/trouble_codes/obd-ii-c-chassis-codes.php

 

[Swims350]

I know what you meant, mine did the same and it was exaclty as I said, bad flex line and rears needing adjusted out.

If they worked fine before and now are junk something blew.

 

[Bigfoot]

I replaced everything on my truck from the master back, calipers, all lines, wheel cylinders, after someone stopped short in front of me. No matter what I did, speed bleed, abs fuse, etc still mushy pedal. I took it to the brake shop and they cycled the abs and bled them and I have good pedal at most. The shop said there is air trapped in the abs module and when the vacuum assist starts it kills the pedal.

 

[duanes7]

Just got back from the stealership. Took it there so they could bleed the abs. I had three codes c0066, c0073, and c0086. The technician didn't even try to run a functional check, just did two squirts of fluid each from all four corners and charged me an hour of labor. Didn't earn their five starts today...

The pedal isn't just soft like if I had a line flexing. It hits the floor and will barely stop the truck on level ground at 10 mph.

I suspect the abs has been worked on before as the one in the truck right now has writing on it like it came from a junk yard.

Brakes worked good from SC where i bought it to here in middle GA. Gave me one hiccup just before I parked it.

Just installed the orig master cylinder and I at least have some brakes. About the last inch seems to work. Can even get the abs to kick in.

I think the booster is working. I pulled and capped the vacuum hose and the pedal pressure didn't die when I started the truck, but I couldn't get the truck to stop either. Also is will splash fluid out of the reservoir like the manual says as one of the booster checks.

Maybe I still have air in the lines. How many pumps of the brake pedal should be enough to get fluid from the master cylinder to the right rear wheel cylinder?