Bad master cylinder, or just a re-bleed needed?

[sciphi]

Today I decided to bed in my new rotors and pads by doing some 60-20 mph stops. The ABS kicked in on the first stop, and the pedal sank a little. On the second stop, the pedal gets firm for a second, then continues down near the floor. Stops after that resulted in a very soft brake pedal, and not much stopping power. The pedal comes back up, only to get near the floor when stopping. The car will stop, as long as there's enough room and I'm standing on the brake pedal. I've tried pumping the pedal with the car off, and it will sink down to the floor. It will come back up like normal with my foot off.

Parts replaced in the past year:
Wheel cylinders
Rear brake shoes/hardware
Front rotors/pads
All 4 rubber brake hoses

Should mention, at the time of the hoses I kicked in the ABS repeatedly on a gravel road. The pedal felt a bit soft right after that, but got better. I haven't re-bled the system since then.

The fluid level looks fine, and there's no fluid in the vacuum hose from the booster.

The steel brake lines are rust-free, as they were replaced a little over a year ago.

What do you folks have to say? Re-bleed, or master cylinder time?

 

[SuperDave456]

I was recently thinking that I was going to have to replace my master cylinder on my '98 F-150 because of a braking issue like yours.

I finally decided to flush/bleed all the brake lines. The first time around did not work. However, when I had my wife start the truck and we bled it that way the issue resolved itself.

I would start with the least expensive option. Bleed first.

 

[oilboy123]

It's an aluminum master cylinder? If so I would say most likely just not bled properly. Try try again. If you have to stand on the brakes it sounds like the front brakes are not even activating?

You bled the brakes the two person method? Or with a vacuum attachment? If you did it with the vacuum try with someone in the car doing the pedal for the last bit on each wheel.

 

[Dave Sherman]

If the brake pedal goes to the floor with very little effort and very little braking power, then that might be air which needs to be bled

If it feels firm but keeps sinking to the floor, that's a major leak somewhere.

 

[CourierDriver]

I dont do brakes, I got to much to loose running freight, I take it to a brake guy who has been doing brakes for 20+ years, ok, I have to throw my ego out the window on that,,yep I let someone else do it, to risky for me. but I dont rebuild trannys, engines,ac unit, and I dont do tune up because getting to the plugs on a van is a nightmare on a one ton dodge. ok,,im done.

 

[KrisZ]

If you think you have a bad master cylinder do the following test.

Start the car, pump the brake pedal few times and apply moderate pressure. You don't have to be moving for this one.

If you apply constant pressure and the pedal slowly sinks to the floor, without you increasing the pressure, you have a bad master cylinder, if the pedal doesn't move down, your master culinder is most likely OK.

 

[sciphi]

It moves a little bit on its own when the car's off and the vacuum is depleted. I'll try re-bleeding it. When my friend and I bled it, he operated the vacuum pump while I pressed the brake pedal down halfway.

Seems odd that it would be firm for a bit, then suddenly get all squishy. We'll see if the bleeding takes care of it.

 

[sciphi]

To update:

I spent the afternoon bleeding the brakes. Bled them twice, once by myself, and once with a neighbor. After a liter of brake fluid, the pedal still sinks to the floor. No air or leaks we can see. Time to bench-bleed the remanufactured master cylinder that was all that was available locally. The system bleed will take place tomorrow when it's back up around freezing.

 

[NJC]

What about isolating the master cylinder by removing and plugging the brakes lines from it - pedal at that point should be rock hard.

 

[sciphi]

Originally Posted By: NJC
What about isolating the master cylinder by removing and plugging the brakes lines from it - pedal at that point should be rock hard.


I removed the old master cylinder from the car already. It's a moot point now...

The old MC had tons of greenish-black brake fluid in it. That's odd, considering every other hydraulic component in the brake system has been redone in the past year, with the exception of the 3 year old remanned calipers. So the fluid is less than a year old, and the stuff in the MC was just a week old. Odd.

 

[sciphi]

Update: The master cylinder was bench bled with all the bubbles out, and the brakes were bled from longest line to shortest. A liter of new brake fluid was run through the hydraulics. The car will stop hard enough to kick on the ABS, but the pedal travel is horrendous. When the car is off and the vacuum depleted, the pedal is rock-hard with a little movement that is within spec according to my FSM's.

Any other suggestions?

 

[sciphi]

Update 2:

Driving around the block very slowly with the brake booster disconnected, the pedal stays high and hard. The car stops just fine with normal pedal travel, just a lot of force needed. As soon as the brake booster is hooked up, the pedal sinks to the floor. Testing the vacuum check valve by blowing into it shows it to only pass air in one direction, as it should.

I'm thinking brake booster.

 

[sciphi]

The brake booster hisses when the pedal is pressed down and held. Plus the MAP sensor reading jumps up when the brakes are depressed. It didn't used to do that.

Ordered a remanned booster, we'll see if this is it.

 

[sciphi]

Bringing this to somewhat of a conclusion:

I installed a remanufactured brake booster. The pedal is still wooden feeling and pretty soft traveling down. On the other hand, it is staying off the floor when pressed, and the car stops very quickly when the pedal is mashed. Or at least it does from 30 mph, the fastest it was tested this evening.

 

[The Critic]

Could it be an issue with the brake lines?

 

[sciphi]

Originally Posted By: The Critic
Could it be an issue with the brake lines?


They're all new. The metal lines are new, and the rubber lines are new. There still could be an issue with them, even being new.

The only brake system components that have not been replaced in the past year are the factory ABS pump, and the 3 year old calipers.

I'll see how it does today when driving to work.

 

[MrBeachcomber]

I'm late to the show on this one as I just read through the thread today. Here's my take after doing 50 years worth of brake bleeding and replacing:

1. Be sure that your emergency brake is off while bleeding the rear brakes if your emergency brake locks the rear drums. (It's hard to get all the air out even with pressure bleeding if the brakes are locked.)

2. The gunk you saw in the M/C was from seal deterioration (more than likely from the disc brakes since they are subjected to the greatest heat cycling during use); however, if your pedal is soft and there's no air in the system, the seals in the M/C may be going too.

3. The vacuum booster only offers a mechanical advantage in stopping the vehicle. If the pedal is lower or not engaging until near the floorboards, you're losing hydraulic pressure somewhere within the system. Leaking caliper seals or swollen/spongy brake lines are two area to check visually. If it's the M/C, most systems will activate the red "brake" line when pressure loss is encountered.

Hope this helps!

 

[sciphi]

Thanks, everybody!

Replacing the booster seems to have been it. The car will throw me into the dash on a hard stop now. Plus, my long-term fuel trims are back around 0. With the old booster they were running about 3-5 cruising down the highway. The car also coasts for a long time now, and the noises from the front end are subsiding. This is after driving the car 50 miles in in-town and high-speed rural driving today.

So, after replacing the master cylinder, the new remanufactured brake booster has cured my problem of a pedal that is hard when the car is off, and to the floor when the car is turned on.