What are the signs of a failing master cylinder?

I've got a brake problem. I've bled and bled my brakes. The pedal will stay hard when the vehicle is off (like my pedal was before), but once I turn the vehicle on the brake pedal has no force and it goes to the floor. Had a couple people tell me to change the master cylinder, what do you guys say?

Need more detail regarding the circumstances surrounding your problem.

first, I've bled my brakes 100 times. If I pump up the pedal when the vehicle is OFF the pedal is hard like a normal pedal. But, once I turn the vehicle on, the pedal will go to the floor. If I put my foot on the pedal with the vehicle off and push down hard (it won't sink far when the jeep is off) the pedal will go the floor once I turn the engine on. That is pretty much the problem. I will have no braking power and there is no way I could lock the brakes up or stop in a quick manner.

It's like I have fred flinstone trying to stop me.

edit: oh and it is a 91' cherokee.

Sounds to me like the diaphragm in the brake booster is leaking or the vacuum line running to the booster is leaking. If the booster is shot, I would upgrade to a late model dual diaphragm booster. Also, make sure the master cylinder isn't leaking into the brake booster causing the diaphragm to fail. Good Luck.

Woody

That was my thought, sounds like a vacuum and or booster issue

What I meant by more detail, is what did you do just before the problem? Were the brakes good one second, then bad the next? Did you just replace pads/linings, and now no brakes? Did you have brake lines disconnected, and now no brakes? Knowing the circumstances associated with your current problem helps identify possible trouble areas. As an example, air in your hydraulic system will give precisely what your experiencing; pedal is hard with engine off, then goes to floor with engine on.

I had replaced a caliper. But, you can only bleed brakes so much before it starts getting crazy. Should I have them professionaly bled or somethin?

Please, folks, let's get the mechanics of power brakes straight here. If the vacuum fails or the booster fails, the pedal will stay hard! Braking effort will be increased greatly, but when a booster fails, the pedal will not sink! If your pedal sinks to the floor, you are getting plenty of vacuum boost. It is more likely that one of three things is happening. Either there is still a problem with bleeding, and the boost is compressing the air in the lines more than you are able to do when unboosted, or there is a problem with the master cylinder, or somewhere a brake is vastly out of adjustment. If you've checked out your brakes and bled them, the master cylinder is the likeliest suspect.

There is no air, vacuum, or hydraulic gap in the mechanical connection between the brake pedal and the master cylinder. The booster is a design that fails safe. If it did not, then you would lose your brakes if the engine stalled.

When the engine is off, there is usually enough vacuum left in the booster for two or three pedal pushes. The pedal will harden up after that. If after leaving the car off for a while, you can push the pedal once and there is still some boost left, your booster is fine.

Here are a couple of things to try. First of all, pump the pedal with engine off until all vacuum is gone. Hold your foot on the pedal, pushing not too hard, but hard enough to hold the pedal down. Start the engine. If the pedal sinks, your booster is working. If it sinks to the floor, try pumping it a number of times (engine still on, boost still occurring). If the pedal rises and feels fairly firm and does not slowly sink down again, look for a very badly misadjusted or malfunctioning rear brake. If the pedal rises and does not sink down again, but feels springy or spongy, look for a bleeding problem. If the pedal does not rise, or sinks slowly under pressure, it's the master cylinder. It's common for a worn master cylinder to hold pretty well under very hard pressure, but to sink under relatively light pressure, because the rubber piston cups are designed to expand outward under pressure, so the harder you push, the tighter they contact the walls of the cylinder. Dual master cylinders are designed so that if one circuit fails the other will still hold, but the holding occurs only at the very bottom of the travel, so if you find some firm brakes, but only near the floor, that is probably the problem. You can test your brakes on loose gravel or dirt, and usually you'll find out right away which circuit is bad, or which wheel is not working.

here is something I noticed. My E-brake is out of adjustment (to where it wouldn't stop me, I assume cause of my 3.5" lift) and in order to increase braking power I had pulled the e-brake partially, which in turn would provide a more solid pedal, and slightly improved braking power (I could now lock up the rear brakes when driving in reverse). I know that it wasn't dragging when I pulled the e-brake because I had pulled off the wheel and examined the drums to make sure.

First, I'll work on bleeding the brakes again and should I assume my rear brake is out of spec?

edit: I know it is late now but I forgot this, it is very important that I mention that when I was bleeding the brakes when all of this happened I accidently let the fluid level get too low air got in the system. I bled the brakes a TON after this, many different days. Should I attempt a different method of bleeding the brakes?

If you sucked air into the mastery cylinder then you may not get it out with traditional pump/release bleeding. The master will keep sucking the air up into it if the bubble is close. You may need to bench bleed the master, or pressure/vacuum bleed it in the Jeep. I'd go with a vacuum bleeder.
What you're sayig about your rear brakes leads me to believe you've got a common problem, the rear adjusters have stopped working.
Here's a link that explains why bench bleeding is important when you get air in te master.
http://www.custompistols.com/cars/articles/dw_benchbleed.htm

87manche said:

If you sucked air into the mastery cylinder then you may not get it out with traditional pump/release bleeding. The master will keep sucking the air up into it if the bubble is close. You may need to bench bleed the master, or pressure/vacuum bleed it in the Jeep. I'd go with a vacuum bleeder.
What you're sayig about your rear brakes leads me to believe you've got a common problem, the rear adjusters have stopped working.
Here's a link that explains why bench bleeding is important when you get air in te master.
http://www.custompistols.com/cars/articles/dw_benchbleed.htm

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I have a mitty-vac and it isn't too useful I think. I'll try it again, but it hasn't worked thus far. Is there anyway to make a better seal around the bleeders when using the vac?

woodyjeep said:

I have a mitty-vac and it isn't too useful I think. I'll try it again, but it hasn't worked thus far. Is there anyway to make a better seal around the bleeders when using the vac?

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Unscrew the bleeder a couple of turns. Take a rubber band and wrap it around the bleeder next to the caliper. Turn the screw in till it feels tight and bleed the brakes. The rubber band will help seal around the bleeder.
I use the mity vac for most of the job, then I get a few strokes the old fashioned way. If I don't do that, some air remains.
If you bled them as many times as you say that you did, I'm willing to bet that one of those times you managed to run the reservoir dry. If so it may be impossible to do a good job without removing the master cylinder and bleeding it.
The master cylinder may not be mounted straight and a buble may be hiding in the MC most likely in the forward area of each compartment.

falcon556 said:

Unscrew the bleeder a couple of turns. Take a rubber band and wrap it around the bleeder next to the caliper. Turn the screw in till it feels tight and bleed the brakes. The rubber band will help seal around the bleeder.
I use the mity vac for most of the job, then I get a few strokes the old fashioned way. If I don't do that, some air remains.
If you bled them as many times as you say that you did, I'm willing to bet that one of those times you managed to run the reservoir dry. If so it may be impossible to do a good job without removing the master cylinder and bleeding it.
The master cylinder may not be mounted straight and a buble may be hiding in the MC most likely in the forward area of each compartment.

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i did run it dry, so it looks like I should take out the master cylinder

not sure about the problem, but when i let my master cylinder run dry i couldnt find a bench bleeding kit ANYWHERE around town.. so i just bled my 1990's ABS master cylinder by the lines going in to the master cylinder using the same technique as regular brake bleeding.. it worked fine? i dont know if you are comfortable trying it like that or not but it worked for me.

AKswampheep said:

not sure about the problem, but when i let my master cylinder run dry i couldnt find a bench bleeding kit ANYWHERE around town.. so i just bled my 1990's ABS master cylinder by the lines going in to the master cylinder using the same technique as regular brake bleeding.. it worked fine? i dont know if you are comfortable trying it like that or not but it worked for me.

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could you expain that more?

"bench bleeding" a master cylinder (MC) while installed can be done, but generally is more difficult. First thing is the MC generally is not level. It MUST be level to properly bench bleed. This could require removing the nuts holding it to the booster and leveling it out; however, it now becomes difficult to operate the plunger. You could bleed it by screwing plugs into both MC outlets, have some one push the brake pedal, and while they hold pressure, you crack open one of the plugs. Insure you tighten the plug before your assistant takes the pressure off of the MC. Repeat this maneuver several times on each plug. Problem with this method (other than the mess it causes) is if the MC is not level, you may still have air trapped behind one or both pistons. I recommend you remove the MC from the truck and properly bench bleed it.

xjbubba said:

"bench bleeding" a master cylinder (MC) while installed can be done, but generally is more difficult. First thing is the MC generally is not level. It MUST be level to properly bench bleed. This could require removing the nuts holding it to the booster and leveling it out; however, it now becomes difficult to operate the plunger. You could bleed it by screwing plugs into both MC outlets, have some one push the brake pedal, and while they hold pressure, you crack open one of the plugs. Insure you tighten the plug before your assistant takes the pressure off of the MC. Repeat this maneuver several times on each plug. Problem with this method (other than the mess it causes) is if the MC is not level, you may still have air trapped behind one or both pistons. I recommend you remove the MC from the truck and properly bench bleed it.

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I don't have access to a vice though

woodyjeep said:

I had replaced a caliper. But, you can only bleed brakes so much before it starts getting crazy. Should I have them professionaly bled or somethin?

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Check out my famous "Another I can't bleed my front brakes thread!"...especially my post #51 on finding the problems:

http://www.naxja.org/forum/showthread.php?t=82430&page=4

Check the caliper that you replaced and make sure the pads are not hung up..."One of the issues was the front passenger side inner pad was bent...I think the brake line must have pulled on the caliper and the inner pad slid off of the guide and hung-up, so when I pressed on the brake pedal it bent the friggin' thing."

Go back to the basics...if you haven't already...I also found on closer inspection my driver side rear brake was rusted frozen. Replaced with all new hardware so there is no doubt it's not working.

I'd suggest get the Speed Bleeders as mentioned in the thread so you don't have to worry about bench bleeding the master cylinder or have the system professionally bleed after you inspect thoroughly one more time.

Good Luck...I'll keep an eye on this thread as I know how frustrating and nerve-wracking this can get.

thanks for the help. This is truely frustrating.

there is no reason to have to bench bleed your master. its a very simple system, and bleeding the brakes will bleed the master. Get it running, and stab the pedal hard and fast. usually a bad master will still work in this manner, slow pressure and it will sink. im willing to bed the master is bad