did I diagnose my brake problem right??

[newbie321]

Hi, All:

Can one make a conclusion that my brake master cylinder is internally leaking based on the following observations...

1) with engine off...the brake pedal is always soft. When I pump it multiple times, it gets harder and harder....BUT if the foot is pushing the pedal, it will slowly, with much resistance go to the floor.

2) when engine is starting....the whatever pressure that was built up in #1 (above) goes to zero and the pedal easily goes to the floor as if it were the knife through the butter.

...here is my hypothesis...
the master cylinder is leaking internally past the secondary or maybe primary cup....I have to apply much force to push the piston further when the engine is off. When hte engine is on, the booster amplifies my force and the piston is pushed much easier.

Can somebody confirm or argue against me as I do not know if I need to replace the master cylinder or if it is ok, but something else is at play...

Thank you,

[dareall]

what you describe could be what is wrong with the master cylinder. To verify this look behind the dash to see if the fluid is leaking inside the car along the firewall or under the hood on the firewall.

what you describe could also be not enough fluid in the master cylinder and/or air in the system. check your fluid level, then bleed the brake lines first and see if this remedies the problem. You may also have a leak somewhere in the system. Wheel cylinders in the rear are usually the first to go. Before you bleed the system check to see that there are no signs of leakage at all 4 wheels.

good luck with this and if you do not know how to do any or all of these things get help or take it to a place that can help. You do not want to get anything wrong when it comes to your brake system.

[mark v]

It can leak internally but not externally, rare but I seen it once.

Suck out all of the fluid in the reservoir and replace it with new fluid. Bleed your brakes to get the new fluid in and past the M/C. This will remove all the moisture soaked brake fluid and you will see if you get a better brake pedal, if it is still bad replace the M/C and do a complete brake fluid flush to all 4 wheels to remove all bad contaminated fluid.

[newbie321]

Hi,

I checked the car for any leaks and I observed no leaks present. So I am proceeding with removing the brake master cylinder and installing a new one....Will post the results once I re-install the part. But it maybe about a week since I am ordering it online, hence shipping time....
Thank you guys for your help and attention!!!

[mark v]

Remember to bench bleed the M/C first then do a complete brake fluid flush on the whole system to avoid and backwards contamination and it also will improve your braking.

Why Change Brake Fluid

Please see above link!

[JRG]

I had a similar problem but still not solved. I had my rear brakes replaced then the pedal became soft. I went back and they bled the brakes and said everything was fine. went to work with my 92 F-250 and almost got in an accident as the brakes didn't respond fast enough. went to another shop and they replaced the master cylinder. Brakes worked good but after stopping the pedal faded to the floor. I went back and they said it was a bad master cylinder and replaced it again. I told them that the problem persisted and they then said it was travel in the air or vacuum system not the master cylinder and told me everything was fine. Well the brakes work but I have never had a pedal fade on me after stopping in all the vehicles I have ever driven. If any one else has an idea what it might be I would certainly appreciate it so I can tell the mechanic what I want done.

[mark v]

Most people will pump a Master Cylinder to bleed brakes, this is a big no no.

Master Cylinders (M/C) will develop a wear pattern in them over the years. When you bleed the brakes and use the M/C and "Pump Them Up" you push the internal piston and O-rings past the wear area and the O-rings get little cuts in them, this leads to loss of pressure and failed M/C.

Most Mechanics will say BS because 1. You spend more money coming back, 2. wont admit they did the damage to make it fail, 3. are uneducated to this problem.

I have personally seen M/C being damaged this way and they always fail right after a brake job after the brakes were bleed.

Think about it..... your bakes never go to the floor when you brake, they only do when you bleed them.

Vacuum bleeding is the only safe way to go. Since I have gone this route I have never had a M/C fail on any car I have done brakes on.

[doug 351]

Originally Posted by mark v

Most people will pump a Master Cylinder to bleed brakes, this is a big no no.

Master Cylinders (M/C) will develop a wear pattern in them over the years. When you bleed the brakes and use the M/C and "Pump Them Up" you push the internal piston and O-rings past the wear area and the O-rings get little cuts in them, this leads to loss of pressure and failed M/C.

Most Mechanics will say BS because 1. You spend more money coming back, 2. wont admit they did the damage to make it fail, 3. are uneducated to this problem.

I have personally seen M/C being damaged this way and they always fail right after a brake job after the brakes were bleed.

Think about it..... your bakes never go to the floor when you brake, they only do when you bleed them.

.

That makes sense- but I haven't done it any other way on several vehicles for many years and never had a master cylinder fail- excepting one that sat for years and it failed without a brake job. I guess I was just lucky.

[mark v]

It really all depends on the wear in the M/C and how quality of a job it was done when it was mfg. The cheaper the metals they use the more wear it gets.

If you have to pump put a hunk of wood under the brake pedal to keep it from going to the floor or further than normal brake travel.

[JRG]

Thanks for the info and on the outside chance that the mechanic did it right, I found this article that is particular to Ford light duty trucks between 87-98......
The Fading Bronco Brake Pedal
who'd a thunk?