Brake Problems

[Ramm]

I have a 1998 Chevrolet Cheyenne, 44,130 miles, 3500 1-ton 4x4 with duals, and a flatbed. The brake pedal goes to the floor with no resistance. Pedal does not like to come back up after it is depressed. Here's what we've done thus far:

1. Replaced the brake lines and bled all four wheels.

2. Replaced the master cylinder after a bench bleed.

3. There are absolutely no visible leaks on the brake lines, attachment points, or on any wheel.

4. We've tried pumping the brakes. We pump the brakes six times, apply light brakes, and start the engine. All of this is futal because the brake pedal goes to the floor with no resistance.

5. The truck came from back East (Georgia) and is exceedingly rusty on the undercarriage.

What's next? Should I replace the boost or replace the wheel brake cylinders which might be bypassing brake fluid? Is there anything I have not considered?

Thank you in advance for reading this post and for any response or opinion you care to offer.

Kindest Regards,
Jim
Sparks, NV

 

[Erik the Awful]

Please forgive me if this sounds demeaning. I don't know your skill level, and from what you've stated you shouldn't be having a problem. That leads me to believe either you got a bad master cylinder or you got something wrong.

First, make sure you have no leaks and all the fittings are tight. Double check to make sure none are cross-threaded. Make sure the reservoir is fully seated into the master cylinder.

Second, check the rear wheel cylinders. They won't "bypass" fluid, they'll either leak, be plugged up, or work.

Third, thoroughly bleed the system and make sure the reservoir never runs dry. If it runs dry you have to start over.

Fourth, if you still don't have a pedal, your master cylinder is bad.

 

[Pinger]

I'd go straight to removing the drums and checking the cylinders for leaks (they hide them well behind the boots) and, that the adjustment on the shoes is in bounds ie, all the pedal travel is failing to bring the shoes in contact with the drums. I'd also be looking for anything broken/dislodged that denies the shoes their travel but allows the cylinder piston(s) to move.
Inevitably you'll find something amiss in the rear brakes. When fixed (or if nothing wrong there) recheck the previous work in accordance with ETA's post above. Might be worth bleeding the MC at the unions and treble checking them for leaks. Air drawn in there when the pedal is released will cause no end of problems.

 

[Schurkey]

Perhaps a moderator will move this out of the "Engine" section and into "Axles + Brakes".

1. Have you bled the ABS unit? You MUST have a scan tool to do this properly. GM recommends using a gallon of brake fluid. (A gallon seems unreasonable to me.) I've posted instructions on a .pdf on this site numerous times; that .pdf is on another computer that I don't have access to right now.

2. Are you SURE there's no air in the master cylinder? Properly bench-bleeding a master cylinder takes more time and effort than most folks realize. They often have air bubbles trapped in the front of the primary and secondary pistons because the master cylinders tend to be tipped "up" in front. Lifting the rear of the vehicle, or unbolting the master from the booster WITHOUT removing the metal brake tubing, and forcing the front of the master "down" then stroking the primary piston "not very far" will allow the air bubbles to transfer to the reservoir via the drilled passages.

3. Have you had the wheels and drums off to verify that the brakes front and rear don't have mechanical issues, and the rear brakes are appropriately adjusted? Calipers float freely on their mounts? Front hubs don't have failed bearings? Rear shoe friction material hasn't fallen off the metal shoe cores?

4. As said, air can leak INTO the brake system even if fluid isn't leaking OUT. This is generally via the drum wheel cylinders, and aftermarket proportioning/combination valves; although any joint in the plumbing is a potential cause.

5. Assuming that this is a hydroboost unit, I'm thinking that a failure in the booster could cause the lack of pedal return...but I'd verify ALL the rest of the brake system, first.

6. A failed ABS unit can cause low brake pedal; again that's not my first suspect.