Brake Booster or Master Cylinder or both?

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westonirons

westonirons

10 Year 15 Year
Joined
Jan 3, 2008
Threads
21
Messages
73
Location
Minnesota
Here's my brake situation:

New stuff in rear, put self-bleeder screws all around and on LSPV. I've bleed and bleed and bleed some more (4 quarts now and have been doing it per instructions on previous threads)...and I'm still getting little bubbles from the LSPV (no bubbles on any of the calipers).

Now I'm thinking the brake booster has gone bad, here's the situation:


When I press on the pedal with the engine off, there is no change in the pedal, all is good. However, when I press on the brake pedal and start the engine, the pedal goes all the way down or close to the bottom quickly.

After the engine has been running for a couple of minutes and shut it off and then press on the brake several times, the pedal gradually rises and feels solid at the top. But, when I press on the brake while the engine is running and stop the engine with the brake pressed down it pops back up.

It's wierd, I have some of the symptoms of a bad brake booster, but not all of them. Makes me wonder the following:

Why do I have some symptoms, but not all?

Does the brake booster need to be replaced or I'm I missing something?

How do you know when the master cylinder needs to be replaced?

Does the LSPV ever go bad? How can you tell?

Is the issue with my self bleeding screws?

Any advice is greatly appreciated. Thanks.
 
Brake bleeding on an 80 can be problematical to say the least.


It appears that the bleeding process can pose hazards to an otherwise servicable master cylinder and/or brake booster.

I have sold several master cylinders to people who can't get a "pedal" after they attempt to bleed and I have sold a few boosters that appear to "fail" during the process. I thought that it was all bull**** until I experienced it with my wife's truck.

In my case I had my 12-year-old son in the driver's seat doing the pedal stuff while I was under the truck. He did what I asked him to do, with gusto. I could NOT get a pedal when bleeding the fronts and I finally surrendered and replaced the master (that was working fine before we started). After I replaced the master and got the pedal back I discovered that the booster was hissing when the pedal was applied...:mad: I then bit the bullet and replaced the booster as well. After that all was well.


Here is what I believe transpired:

Remember that we are dealing with a 12+year-old vehicle and the brake master/booster package is used to traveling in a repeatable distance. That means that there is a very specific wear pattern in the cylinder bore and in the booster bore.
Agressive bleeding will over-travel both the master and the booster. If there is any "garbage" in the area where the over-travel ends up it will tear-up the seals in both components ruining them in the process.

I really believe that this is the ultimate cause.

In view of this I recommend that bleeding the brakes on an 80 should be done with "gentile" pedal usage in order to prevent damaging the seals.
 
Monday.
 
cruiserdan said:
Brake bleeding on an 80 can be problematical to say the least.


It appears that the bleeding process can pose hazards to an otherwise servicable master cylinder and/or brake booster.

I have sold several master cylinders to people who can't get a "pedal" after they attempt to bleed and I have sold a few boosters that appear to "fail" during the process. I thought that it was all bull**** until I experienced it with my wife's truck.

In my case I had my 12-year-old son in the driver's seat doing the pedal stuff while I was under the truck. He did what I asked him to do, with gusto. I could NOT get a pedal when bleeding the fronts and I finally surrendered and replaced the master (that was working fine before we started). After I replaced the master and got the pedal back I discovered that the booster was hissing when the pedal was applied...:mad: I then bit the bullet and replaced the booster as well. After that all was well.


Here is what I believe transpired:

Remember that we are dealing with a 12+year-old vehicle and the brake master/booster package is used to traveling in a repeatable distance. That means that there is a very specific wear pattern in the cylinder bore and in the booster bore.
Agressive bleeding will over-travel both the master and the booster. If there is any "garbage" in the area where the over-travel ends up it will tear-up the seals in both components ruining them in the process.

I really believe that this is the ultimate cause.

In view of this I recommend that bleeding the brakes on an 80 should be done with "gentile" pedal usage in order to prevent damaging the seals.
Click to expand...

great post, totally agree. Had a reman'd unit blow a seal during the bleeding process....actually at the end of the pedal pump 2-person bleeding process!

I've never had any luck with those self/speedy bleeders. It seemed that air would get past the threads on the ones I had, very frustrating experience especially since it was a pain to locate them and i had also drank the koolaid. :D

The best luck I've had bleeding the 80 is with pressure bleeding, I use the Motive unit but one can be made if you have the time and want to save a few bucks. I was able to skip the bench bleed, although I did do a gravity bleed on the MC before hooking up the pressure bleeder.
 
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I would start with replaceing the master cylinder first and see if that works.Boosters are $$ and usually give a firm hard brake pedal, 2 ents Mike
 
I'm having a similar experience with my brakes. Low pedal when running. With the engine off that first push on the brake pedal is just as low but the second one is at the top again. The MC is only a year or so old but he booster is original. I'm having a hard time understanding how the booster could do this.
 
What test can I perform to know if my master cylinder is toast? I have recently replaced all four rotors pads, LSPV, rebuild calipers and replace all of the rusted lines on the axles with stainless and added Slee's braided lines. I first did a standard bleed with my wife at the at the pedal and put a block of firewood under it so I wouldn't of extend the travel of the MC. I knew there might be some air left so I bought a Mityvac and went through two quarts of fluid I can't believe how much air was in all of the lines!!! So I'm at a loss myself! Any help or suggestions will be greatly appreciated.
 
i think your booster is fine westonirons. the symptoms you described is just air still in your brake system. if pedal goes to floor slowly while engine is on, there must be a brakefluid leak somwhere. check for leaks. even the smallest leak could give you these symptoms. what i do is i wash every brake line with water then dry it up clean then pump the brakes. you could easily spot small leaks after this:wrench:
 
I think, and correct me if I am wrong, that my brakes work fine summer/warmer months. When winter hits, I start the car and I have no brakes. I hit the pedal and it pegs down to the floor. I have to pump it a few times and then all is well. But cold start = no brakes. That is a brake booster problem I think right?
 
westonirons said:
What about the LSPV, how can you tell if it's bad or not? Do you simply look for leaks or is there a test for it?
Click to expand...
i dont know much about the LSPV. i think you just look for visual leaks. nothing much to service in that part. how about trying to bleed the rear right, LSPV, rear left, LSPV again, front right, then front left calipers in that order. right after finishing off with the calipers, bleed the master cylinder. this worked wonders on my hdj81. because i still found air in the master even after bleeding all calipers. but after bleeding the master it worked great. i almost thought it was hopeless till i tried this
 
slowgrowler said:
I think, and correct me if I am wrong, that my brakes work fine summer/warmer months. When winter hits, I start the car and I have no brakes. I hit the pedal and it pegs down to the floor. I have to pump it a few times and then all is well. But cold start = no brakes. That is a brake booster problem I think right?
Click to expand...
this only happens during start-ups? the diaphram in the booster might be binding up when freezing cold. does your booster have a check valve in series with the hose? might be sticking too.
 
Are there any good instruction on how to install a new MC and brake booster? How does bench bleeding work? Any tips on the install would be greatly appreciated. I've read the FSM and I'm not sure how I'm going to set the mc piston setting. Thanks for sticking with me on this.
 
Let someone else give it a shot by pulling a vacuum on it.Sometimes all it takes is a little help I know sometimes friends see something I have missed. 2 cents Mike
 
re: Michael Hanson

Because I can move the pedal (engine on or off) you think the booster is fine?

It was weird. I was bleeding the lines getting bubles and a steady stream, then nothing. The pressure when bleeding was gone. The pressure came back after a whle, but wasn't the same, firmed up again, then nothing. If the pedal goes all the way down, I may see a small amount come out. I'm thinking it's the master cylinder. I've been bleeding for like five or six quarts now. Also, I don't see any leaks at all.
 
westonirons said:
...When I press on the pedal with the engine off, there is no change in the pedal, all is good. However, when I press on the brake pedal and start the engine, the pedal goes all the way down or close to the bottom quickly.

After the engine has been running for a couple of minutes and shut it off and then press on the brake several times, the pedal gradually rises and feels solid at the top. But, when I press on the brake while the engine is running and stop the engine with the brake pressed down it pops back up...
Click to expand...
with how you described the situation, i think your brake booster is working fine. when engine on, the pedal is supposed to feel softer, and with engine off, the pedal is suppsed to get stiffer. i think you should change your master cylinder or restore it with a repair kit. the rubber cups might be worn down. giving you pressure at times and not most of the time
 
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