Brake! Warning on '89 K1500

[johnsonmtz]

I have an '89 K1500 with a constant BRAKE! warning on the dash. The truck had the classic mushy brake pedal and I read you can replace the OEM master cylinder with a unit from an '03 that will solve that issue. I made that swap a couple years ago and the pedal became very stiff after that, but the brakes worked okay.

Flash forward a couple weeks ago and I blew the rear line followed by a BRAKE! warning on the dash and fluid on the ground. This repair turned into replacing every line from the MC all the way to each rear wheel cylinder. I bled all the lines using my Motive Gear power bleeder, the BRAKE! light went away and we are back to a stiff pedal with working brakes. BTW, the rear brake hardware and drums were all replaced about a year ago.

Yesterday the BRAKE! warning came on again with no other apparent issues: Fluid level in the MC is fine and no leaks anywhere. I'm thinking this is either an issue with my ABS valve or the proportioning valve. Since I have a very stiff pedal I'm also wondering if the vacuum booster isn't working right and needs replaced as well. I don't hear any vacuum leaks in the booster, but the pedal pretty much feels like manual brakes with zero vacuum assist. The booster is 34 years old now, so I would imagine it's probably not a bad thing to replace it, but beyond the booster, are there any guides for diagnosing the valves?

I would appreciate suggestions where to start on my troubleshooting.

Thanks.

 

[Schurkey]

1. Which brake option code do you have? You almost certainly have the WRONG master cylinder, incompatible with your low-drag front calipers. This may explain why your power booster seems ineffective--the master cylinder bore is just too huge.

2. Did you bleed the RWAL unit? In '89, it should be iron-bodied with it's own bleeder valve.

3. The BRAKE warning light does three things. Comes on with the park brake, comes on with the safety switch in the brake hydraulic system, and comes on with the ABS to show faults.

4. The leading-trailing shoe brakes on "most" 1500 pickups are a mess. Be sure they're properly adjusted, and that you use the park brake frequently. If this is not a pickup, be sure to specify.

 

[Erik the Awful]

The '03 master cylinder does not belong on your truck. It will give a great pedal feel, but in an emergency stop you won't be able to get full braking force.

Put the right master cylinder on, bleed thoroughly, and adjust your rear drum brakes.