Quick master cylinder question
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
How important to the booster operation are the gasket between the booster and firewall and the rubber gasket/seal/thingy that fits in a groove on the back of the master surrounding the piston where the push rod contacts the piston. The booster is a sealed unit so I would't think they would have anything to do with the vacuum operation of the booster. My new master did not come with the rubber thingy on the master so I used the old one. BTW, EasyEd is welcome to chime in on my thread as long as we are talking about the same problem. We need some booster gurus to chime in also. I think only Roger offered a cause for this ... still air in system.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
I've been trying to think how to explain why the booster reacts the way it does.
Let me say this.
If I take a perfectly functioning brake system , then let a little air into one caliper,bleed off the vacuum reserve in the booster,then step on the brake pedal , the pedal will be high and hard,then start the engine,step on the pedal , the pedal will be on the floor.
If this is the way your cars are acting I doubt the booster is your problem.
Let me say this.
If I take a perfectly functioning brake system , then let a little air into one caliper,bleed off the vacuum reserve in the booster,then step on the brake pedal , the pedal will be high and hard,then start the engine,step on the pedal , the pedal will be on the floor.
If this is the way your cars are acting I doubt the booster is your problem.
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
That explains it perfectly, thats exactly what mine is doing. I was wondering where all of the brake fluid was going when I kept pressing the pedal to the floor with the engine running after I had the full and hard pedal prior to starting the engine. I guess it has to be air still in the system compressing. After the replacement master comes in on Tuesday I guess I'll be back bleeding again. Roger, with all new parts such as my system how many bleed sessions of all four wheels should it take to expel all air, of course after bench bleeding the master throughly. This would be the pedal method exactly as you previously recomended.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
2 times around usually produces a darn good pedal. I fear you might have air spread throughout your system which makes it more difficult. When the air in the system is all together as 1 big bubble it moves through easily but when its broken up it becomes a real challenge. I made a Motive type many years ago and use it for cases like yours but generally I just gravity bleed assisted by my 3/4" wrench , tap tap tap tap tap. 
Instructor
Joined: Aug 2010
Posts: 129
Likes: 0
Billy,
Thanks for letting me in on this. It's nice of you.
I just got back from the auto parts store where I picked up a new check valve and gasket, as well as power steering fluid since I didn't want to run the car with an empty reservoir. That is another story in itself. I figured the cheapest easiest part was a place to start.
Check valve may have made some small difference with the car running, but now the pedal is mushy with the car off and the booster free of vacuum.
Roger, looks like you are right, as usual, and there is still air in the lines. Maybe all the monkeying around has gotten it somewhere it wasn't before, or there is an new leak somewhere. Rats, rats, rats!
I'll have the front end up on blocks to work on a power steering valve rebuild. Can I gravity bleed both fronts at the same time if I keep a close eye on the master so it doesn't run dry?
Ed
Thanks for letting me in on this. It's nice of you.
I just got back from the auto parts store where I picked up a new check valve and gasket, as well as power steering fluid since I didn't want to run the car with an empty reservoir. That is another story in itself. I figured the cheapest easiest part was a place to start.
Check valve may have made some small difference with the car running, but now the pedal is mushy with the car off and the booster free of vacuum.
Roger, looks like you are right, as usual, and there is still air in the lines. Maybe all the monkeying around has gotten it somewhere it wasn't before, or there is an new leak somewhere. Rats, rats, rats!
I'll have the front end up on blocks to work on a power steering valve rebuild. Can I gravity bleed both fronts at the same time if I keep a close eye on the master so it doesn't run dry?
Ed
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
So, what we need is a booster Guru. Billy's problem and mine diverge at this point. His booster is new and, unless he forgot the firewall gasket or didn't torque the bolts (No offense intended Billy) there should be no reason for it to leak. Unless it it a defective unit. Mine on the other hand is probably the original that came on the car. An ugly rusty thing.
How then to proceed without throwing a lot of wasted money at the problem? I have searched the forum and found little about booster problems.
Starting at the front, the check valve and gasket seem fine. There is no gasket between the M/C and booster except for a rubber washer that wraps around a copper (?) washer. Rubber side out. Could that be in backwards? I can't find that part in any catalog.
If the diaphragm(s) is shot, or the air valve is sticking, then it would mean replacing the booster. If the firewall seal is leaking, I might as well replace the booster too, since I would have to pull the unit to do the job. Sounds like a Mother to get to the nuts.
Last, there is a rear air filter, but the GM FSM only says it can be removed, shaken out, washed, dried and reinstalled. Do you have to pull the booster to do this?
So it sounds like I am replacing the booster. Last question and I will stop my hi-jack. Do I just replace the booster, or should I replace the master too as a matched set? From who?
Thanks,
Ed
How then to proceed without throwing a lot of wasted money at the problem? I have searched the forum and found little about booster problems.
Starting at the front, the check valve and gasket seem fine. There is no gasket between the M/C and booster except for a rubber washer that wraps around a copper (?) washer. Rubber side out. Could that be in backwards? I can't find that part in any catalog.
If the diaphragm(s) is shot, or the air valve is sticking, then it would mean replacing the booster. If the firewall seal is leaking, I might as well replace the booster too, since I would have to pull the unit to do the job. Sounds like a Mother to get to the nuts.
Last, there is a rear air filter, but the GM FSM only says it can be removed, shaken out, washed, dried and reinstalled. Do you have to pull the booster to do this?
So it sounds like I am replacing the booster. Last question and I will stop my hi-jack. Do I just replace the booster, or should I replace the master too as a matched set? From who?
Thanks,
Ed
2 Two different symtoms
3 The dust boot flange acts as the firewall seal
4 Because of 3, you have to pull the booster and if you want the booster to react faster, the filter can be left out or cut down the foam.
5 Booster isn't bad if you do it properly. You can also replace booster without disconnecting brake lines and needing bleeding.
If master is good, then just replace the booster. Do all the brakes test to make sure that it's bad.
Years ago kits were available from Bendix or repackaged by GM, but now parts are only available to dedicated certified booster rebuilders.
It is not that uncommon to get defective boosters as well as rebuilt masters.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
I've been trying to think how to explain why the booster reacts the way it does.
Let me say this.
If I take a perfectly functioning brake system , then let a little air into one caliper,bleed off the vacuum reserve in the booster,then step on the brake pedal , the pedal will be high and hard,then start the engine,step on the pedal , the pedal will be on the floor.
If this is the way your cars are acting I doubt the booster is your problem.
Let me say this.
If I take a perfectly functioning brake system , then let a little air into one caliper,bleed off the vacuum reserve in the booster,then step on the brake pedal , the pedal will be high and hard,then start the engine,step on the pedal , the pedal will be on the floor.
If this is the way your cars are acting I doubt the booster is your problem.
If there is air in the hydraulics, it will be compressed and the pedal won't be as hard and it will be lower.
Billy,
Thanks for letting me in on this. It's nice of you.
I just got back from the auto parts store where I picked up a new check valve and gasket, as well as power steering fluid since I didn't want to run the car with an empty reservoir. That is another story in itself. I figured the cheapest easiest part was a place to start.
Check valve may have made some small difference with the car running, but now the pedal is mushy with the car off and the booster free of vacuum.
Roger, looks like you are right, as usual, and there is still air in the lines. Maybe all the monkeying around has gotten it somewhere it wasn't before, or there is an new leak somewhere. Rats, rats, rats!
I'll have the front end up on blocks to work on a power steering valve rebuild. Can I gravity bleed both fronts at the same time if I keep a close eye on the master so it doesn't run dry?
Ed
Thanks for letting me in on this. It's nice of you.
I just got back from the auto parts store where I picked up a new check valve and gasket, as well as power steering fluid since I didn't want to run the car with an empty reservoir. That is another story in itself. I figured the cheapest easiest part was a place to start.
Check valve may have made some small difference with the car running, but now the pedal is mushy with the car off and the booster free of vacuum.
Roger, looks like you are right, as usual, and there is still air in the lines. Maybe all the monkeying around has gotten it somewhere it wasn't before, or there is an new leak somewhere. Rats, rats, rats!
I'll have the front end up on blocks to work on a power steering valve rebuild. Can I gravity bleed both fronts at the same time if I keep a close eye on the master so it doesn't run dry?
Ed
Ir's only there to provide a brake application or two if the engine fails and to prevent a backfire going to the booster.
Mushy pedal with no vacuum is air or a failing master.
If you're having a lot of trouble bleeding, sometimes it helps to drive the car for awhile to try and work any air around and also to help seat the pads better to the rotors, every bit can help.
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Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
I've been trying to think how to explain why the booster reacts the way it does.
Let me say this.
If I take a perfectly functioning brake system , then let a little air into one caliper,bleed off the vacuum reserve in the booster,then step on the brake pedal , the pedal will be high and hard,then start the engine,step on the pedal , the pedal will be on the floor.
If this is the way your cars are acting I doubt the booster is your problem.
Let me say this.
If I take a perfectly functioning brake system , then let a little air into one caliper,bleed off the vacuum reserve in the booster,then step on the brake pedal , the pedal will be high and hard,then start the engine,step on the pedal , the pedal will be on the floor.
If this is the way your cars are acting I doubt the booster is your problem.
What am I misreading?
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
What I've always experienced is when there is air in the system the pedal is not as high and hard as it is with no air in the system.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
True when the engine is "running" but what were talking about is when the engine is not running. With the engine not running it gives you a false sense of a high hard pedal until the engine starts ,vacuum builds and then the pedal goes to the floor.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
completely, but it's not going to be as high as if there was no air in the system. Air compresses leaving a lower pedal, that's all.That's the problem with this internet diagnose stuff.
One person's description may be completely different than another's.
Normally oem should not have more than 3/4 to 1" pedal travel, but someone may have 2" and describe it a high and hard pedal, where in reality it has air.
Example is this thread about the fuse link
http://forums.corvetteforum.com/c3-t...with-fuse.html
One poster missed that it was not protecting a 10ga.
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
My replacement master cylinder arrived from Tuff Stuff late this evening. I've bench bled it already, no bubbles at this point. I'm letting it set with the bleed tubes still installed and plenty of fluid in the m/c. I'll bleed it some more tomorrow to make sure there is no air left in it then I'll install it on the car and try gravity bleeding the calipers. Keep your fingers crossed ..... again !!
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
I bench bled the new replacement master cylinder a bunch of times over a couple of days. I'm absolutly positive there is no air in it, period. Talked to a "TOP GUN" tech at the Chevy dealership that I was at buying parts for another car and told him the problem with my brake pedal going to to the floor with the engine running. He was as baffled as everyone else. His only answer was to try power bleeding the brakes at the dealership. He said it couldn't be the booster because the shaft going through the booster to the m/c push rod was solid so when I push on the pedal the push rod has to move the m/c piston. To isolate the master cylinder and booster from the rest of the brake system he suggested putting the m/c on the car after bench bleeding it and putting solid plugs in the two brake line ports. Push the pedal down and the pedal should be full and hard, then start the engine and push the pedal down again, the pedal should remain full and hard. I did as as he instructed, the pedal was full and hard , I started the engine and pushed the pedal down and the pedal slowly went to the floor. WTF Can someone please explain what's happening here? Wanna buy a car cheap ..... nooooo I didn't say that .... did I.
Le Mans Master
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From: mount holly NC
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Sorry to hear that...
You've tried everything else...seems like its time for a new booster.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
I bench bled the new replacement master cylinder a bunch of times over a couple of days. I'm absolutly positive there is no air in it, period. Talked to a "TOP GUN" tech at the Chevy dealership that I was at buying parts for another car and told him the problem with my brake pedal going to to the floor with the engine running. He was as baffled as everyone else. His only answer was to try power bleeding the brakes at the dealership. He said it couldn't be the booster because the shaft going through the booster to the m/c push rod was solid so when I push on the pedal the push rod has to move the m/c piston. To isolate the master cylinder and booster from the rest of the brake system he suggested putting the m/c on the car after bench bleeding it and putting solid plugs in the two brake line ports. Push the pedal down and the pedal should be full and hard, then start the engine and push the pedal down again, the pedal should remain full and hard. I did as as he instructed, the pedal was full and hard , I started the engine and pushed the pedal down and the pedal slowly went to the floor. WTF Can someone please explain what's happening here? Wanna buy a car cheap ..... nooooo I didn't say that .... did I.
Hydraulics are powerful. The system with no leaking seals can easily balloon flex lines and even expand hard brake lines to a small degree, but that adds up in their total length. It doesn't take much distortion to deplete the volume contained in the stroke of the master (approx 1-1/4").
In 35+ years I've only run into similar problems about 5 times. More common is just a ruptured diaphragm and a very hard pedal with engine on. The last similar problem I ran into, when the engine started, the pedal went down to the floor all by itself. Brakes were fully applied by themselves. Defective booster. Didn't even touch the hydraulics to replace and had only 2" pedal down with engine on and brakes fully applied hard.
You may try loosening the master from the booster approx 1/16" and testing. You may now be putting too much pressure on the reaction disc inside the booster keeping pressure on the internal air valve.
Otherwise replace the booster with a known good unit.