Brakes not working O_o

So a few weeks back I discovered that my brakes are not working properly and I need to get it fixed asap.

Background: Front pads and rotors are just fine and were replaced less then 1 year ago. System was bleed less then 1 year ago as well. Rear drums and shoes still have life left in them. Unknown how long problem has existed since there is no way to tell something is wrong unless you "slam" the brakes. There are no leaks whatsoever and fluid levels have not decreased at all.

Issue: Brakes work perfectly fine and stop the truck under normal driving and braking conditions. However, during higher speeds, if the pedal is "slammed" to the floor, braking power is not sufficient. When you slam the brakes the truck slows as if you are only applying about 50% brake power. Anything past ~50% pedal makes no change in the amount of braking power or force being applied. The wheels do not lock up, the ABS does not engage because there is not nearly enough force to get the wheels close to locking up. Stopping distance is drastically increased and pedal feels spongy and sinks to the floor.

I have determined through basic troubleshooting that the booster is just fine. I believe either A: My master cylinder is going bad or B: I have air in the lines.

Short of bleeding the entire system just to eliminate air as a possible issue, can any experienced mechanics chime in on what this could be? I just dont see how air could be in the lines since the last system bleed, unless somehow air is in the master, but i dont know how to diagnose that without doing a bench bleed and reinstall to check. Just not sure if i should waste time bleeding lines first, then bench bleed the master, then last resort replace the master OR if i should just say screw it all, fork up the money for a new master and replace it along with a full system bleed all at once.

Feedback is appreciated!

 

I would go with the easy and cheap fix first. When was the last time you flushed the brake fluids? Usually when you overheat the pads or boil the brake fluids...they are no longer good. You have to replace brake fluid ASAP. Have you checked if the pads are glazed? Use some sand paper and sand the glaze off the pads. Last I would give the brake system a through brake flush if you have already cleaned the pads and didn't boiled the brake fluid. All the moisture through the years will make the brake fluid less effective. Also you can always check the master cylinder with a vacuum test to see if the it is working correctly. If it's not holding the correct amount of vacuum then replace it.

 

Had one at work that was doing something similar but conditions could be duplicated at low speeds also. Depending on how you pressed the pedal it would either work perfectly or go to the floor and eventually stop.

We put two aftermarket masters in it with same problem. Factory master fixed the issue.

 

Brakes system was flushed and bleed less then 6 months ago. Pads were replaced roughly 6 months ago and rotors were not replaced. I can rough up pads (and rotors??) And try that. I can also replace fluid again but i do 90% freeway driving and dont ride the brakes at all.... i would think its nearly impossible to boil the fluid or glaze the pads in the normal driving conditions and in such a short period of time. How would you vacuum test a master?

 

If i end up replacing the master ill go oem. Aftermarket masters are around 120ish for a good one and i can get a brand new oem master for about 165 so its not too much different

 

how does the brake pedal feel when pressing to the floor and holding it steady without the motor running?

 

Does the pedal actually go to the floor or does it not?

If it goes to the floor you either have air in the lines or a defective master cylinder.

If it doesn't go to the floor but is not effective you have a defective brake booster.

You say you have ruled out the brake booster as a cause. How did you do that?

 

Pedal gets stiff when depressing it while the engine is off. The more you pump the more firm it gets. This is mainly how i know the booster is working.

Pedal goes completely to the floor with fair to moderate pressure at any speed even stopped.

See above for booster info

 

You'll also need to hold it steady for a minute or 2 when motor not running and not just for a few seconds

 

It sounds reasonable to assume either you have air in the brake lines or the master cylinder is leaking by internally. This is assuming that you do not have a leak in the brake system.

Before replacing the master cylinder I would get a vacuum or pressure brake bleed done. Then if you still have the same symptoms replace the master cylinder.

 

One more suggestion.

Inspect your brakes. I don't care if it was worked on a week ago. If you are losing pressure, you either have air in the system or there is a leak.
Leaks can be hidden behind boots, especially on master cylinders. Look for any wetness under your carpet, and around the push rod especially. Pull back the boot if you can.
Check for wetness around your calipers and if that is a no go, look at the back of your drums. A blown wheel cylinder can hide a leak for a long time within it's boots.
Air gets into the system because of leaks. You lift on the brake and it sucks air through the leak back into the line making the problem worse.
It is also possible to have a damaged or loose bleeding screw. Check that they are all tight during your inspection.

Boiling brake fluid is a possibility, but unlikely unless you tow a great deal or rally your truck.

EDIT: that is if your master cylinder is working. Your booster is working, and when you stand on it you force what ever seal that's leaking to give up the ghost. If you can't find any sign anywhere of leaking fluid then it is definitely your master.

 

Ill try this again. It should remain stiff correct?

Ill look into this vacuum test. Is this something ill need to have a shop do or i can do it?

I actually did forget to mention that i have done much of this. Pulled the drums and inspected for a possible blown wheel cylinder. No signs of oil in there. Looked under carpet too. Checked calipers and all around the master and booster. Cant seem to find anything. And the fluid level hasn't changed at all since this stated. Ill do another pass at everything again to be sure though.

 

Then it sounds like you have about narrowed it down to the master cylinder. It sounds as if the piston is allowing blow-by when under pressure, or it wasn't properly bled before being introduced to the rest of your system. The thing is though that as far as I know about master cylinder construction, if you do have blow-by then eventually the outer seal will fill up. Either it finds a way out, or you will basically become hydro locked.

 

One more thought. If you have a manual then the clutch master cylinder gets it's fluid from the brake master cylinder reservoir.
I doubt it could introduce air into the master cylinder though, but it is another related system.

EDIT: nvm, I see that you have an auto.

 

For the most part it seems my suspicions of the master cylinder are confirmed. Ill likely just replace the master with a new oem one and bleed the whole system at the same time. Truck is just shy of 110k so i guess its not unreasonable

 

It isn't a test.

It is a method to bleed the brakes either using a vacuum to suck fluid through the lines. Or there is also a method to use pressure to push fluid through the lines.

 

Gotcha that part i get i thought i read to vacuum test the master to check if its holding pressure. That's where i was lost. But a vacuum flush i can make happen.

 

You could also have a leaking dump valve in the ABS HCU.

 

Not saying it isn't possible, but highly unlikely.