Brake problems

[Randy Bush]

So on my 93 IDIT dually I have the sinking pedal problem, brake solid with engine off. Fairly new rear shoes, adjusted. Have replace master cylinder in past. bleed system. ABS light is on and off , more on. Solution? Replace RABS valve? bypass? Am trying to get this truck ready to sell. Would like to fix the problem right though.

Thanks

 

[riphip]

Make sure it is not your vacuum pump/low vacuum.

 

[Cubey]

Sounds like vacuum pump.

My "rear antilock" light comes and goes. Folks here say rear brakes need adjusting every few months but its been almost 2 years since they have been touched and braking seems fine.

The rear brakes like to lock up at verly low speed only when I first start out driving, before they get warmed up. After just a couple minutes, they are fine.

 

[Randy Bush]

Make sure it is not your vacuum pump/low vacuum.

New vacuum pump installed. Pedal will sink all the way to floor if you push ******* it. Stops just fine if lightly pushed.

 

[Thewespaul]

Did you bleed the rabs valve?

 

[JakeUlch]

Leaking any brake fluid?

 

[Randy Bush]

Did you bleed the rabs valve?

Never did , bleed everything else until was clear, guess maybe give that a try.

 

[Randy Bush]

So here is what I have done to it. bypassed the Rabs hyd valve. no change. tried a value off of a dead truck , nope, put a new booster and mastercylinder and have bleed the brakes until all was clear. Adjusted rears. Nothing has changed the pedal drift. Only thing have gained is the ABS light is no longer on.

 

[Cubey]

What about the hydraulic hoses? Maybe they are ballooning under pressure.

 

[Randy Bush]

have been replaced not to long ago.

 

[Thewespaul]

Randy I would build a brake pressure tester out of some brake hose and a high psi hydraulic gauge. You can screw in your gauge in place of each brake hose and test what’s getting pressure when. It could be the are getting pressure much later than your fronts which is causing the sinking pedal, I fixed this same issue with my 93, fix ended up being a different brand master cylinder and an adjustment on the rod to take up some slack in the pedal box.

 

[gfemling]

“The rear brakes like to lock up at very low speed only when I first start out driving, before they get warmed up. After just a couple minutes, they are fine.“
Curious what “experts” here know about this- I have same but after first few events all works fine. Usually more apparent with trailer and marginal traction- like wet pavement. Suspecting rear tires require a minimum amount of traction friction to break the rear brakes loose again when RABS valve opens but no idea if this is right? Others thoughts?

 

[Thewespaul]

If everything works fine after the first events then Id say look into replacing the shoes. Contaminated shoes (from leaking wheel cylinders) will act this way

 

[gfemling]

Pulled rear brake drums and linings not contaminated so thanks for the suggestion but not the problem here.

 

[Randy Bush]

Although have not tried it yet on mine, I am thinking of getting some plugs to put in the mastercylinder to confirm good no good on that even though it is new.