Spongy brakes after bleeding and new master cylinder.

[Aarondash]

I got a 1998 k1500. When I bought it the brakes were very soft. The truck was sitting for about two years and who knows when the fluid was last changed. I bled the whole brake system. After that they felt slightly better but I still have to push the pedal to the floor to stop. So I changed the master cylinder and rebled the brakes. The brakes again feel a little bit better and I don’t have to slam to the floor but they still are spongy. Any idea why it keeps feeing spongy?

 

[df2x4]

Have you bled the ABS module using a scan tool? If not, that would be my first step.

 

[1997]

also check rear brake adjustment ....

 

[Aarondash]

Have you bled the ABS module using a scan tool? If not, that would be my first step.

uh how do you do that?

 

[df2x4]

uh how do you do that?

Step one is either buy a GM Tech 2 or equivalent bi-directional scan tool that can do the auto-bleed procedure, or take the truck to someone who has one. More info here:

http://67-72chevytrucks.com/vboard/showthread.php?t=782499

The procedure is most likely detailed in the factory service manuals as well, which you can download here for free.

https://www.gmt400.com/threads/88-98-service-manuals.43575/

 

[Schurkey]

Have you bled the ABS module using a scan tool? If not, that would be my first step.

also check rear brake adjustment ....

Thanks. You guys saved me a lot of typing. The scan tool, and the horse-crap Leading/Trailing shoe drum brakes on the 1500s is about 80% of the "squishy/low brake pedal, no stopping power" complaints on these vehicles.

The only things I'd add to that is that many folks don't bench-bleed the new master cylinder long enough; or they try to do it on-the-vehicle where the front end of the master is tipped "up", trapping an air bubble. Front of the master needs to be level, or preferably tipped "down" a few degrees so the air moves to the rear of the master, then vents out the ports between the cylinder and the reservoir.

There is the fairly-remote possibility that one or more of the accumulator valves in the ABS unit is stuck open. Yeah, it can happen. Yeah, it's pretty rare. When an accumulator valve is stuck open, the brakes will act like there's air trapped inside the plumbing. But usually, there REALLY IS air trapped inside the plumbing...somewhere.

uh how do you do that?

See attached .pdf. Your truck would be the second page. Two procedures, but both REQUIRE a scan tool that can chatter the ABS solenoid valves. And after the ABS is bled, you still have to bleed the wheel cylinders/calipers again to remove the air that was in the ABS unit but is now in the plumbing to the wheels. And all this assumes that the master cylinder was properly bled to begin with.

The "Low pressure" method supposedly requires a "special tool" to depress the stem of the metering valve. You can probably get by without that tool. You will need it only if the front brakes don't bleed properly--lack of fluid flow. My pressure bleeder is aired-up to about 10--15 psi, I don't have trouble with the metering valve closing. At higher pressure, the metering valve may have to be forced open with the special tool. At very high pressure--like pushing the brake pedal with your foot--the metering valve gets blown off it's seat and flows again. So the "special tool" is only needed for medium-pressure bleeding of the FRONT brakes.

 

[Erik the Awful]

Did you bench-bleed the master cylinder before you installed it? If not, you have a lot of leg presses to do on that brake system before it gets better. Rear drum adjustment is also critical. If the adjustment is loose, you'll have a lot of free pedal before the brakes even start to engage.

Adjust the rear drums.
Bleed the ABS.
Bleed the rest of the brake system thoroughly.