Quick master cylinder question
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
Quick master cylinder question
My brake pedal goes to the floor. I have bleed the brakes too many times to count and can get half a pedal. As soon as I get the wheels back on and before I even drive it the pedal is on the floor again. Master clyinder remains full of fluid and there are no leaks anywhere. I think it is a bad master cylinder. It is a brand new chrome one from Tuff Stuff and yes I bench bleed it for a long time, no bubbles at all. I just unhooked the brake lines and pluged the port holes with brass plugs. When I depress the pedal it still goes to the floor. Has to be the master cylinder is bad. What do you guys think? I just ordered a rebuild kit for it and it is supposed to be here tomorrow. Is there any other way to test the master cylinder before I remove it. 
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
My quick question is turning into a little more depth. My master cylinder rebuild kit that I ordered did NOT come in today so I removed the m/c anyway. I decided to try bleeding it just for fun. I clamped it in the vice and started bleeding it. The fluid level rises about 1/4 inch when I push the piston in and goes back down when I release the piston. A few bubbles came through the clear lines at first. I can see the fluid going through the clear bleed lines. Sooooo, I'm not sure if the m/c is bad or not. As I previously stated, when I disconected the brake lines yesterday and installed brass plugs in the ports the pedal would still go to the floor so we thought the m/c was bad but could it be that it had air in the m/c and would that cause the pedal to go to the floor as described. I don't want to rebuild it if its not bad. How can I know for sure?
Race Director
Joined: Mar 2006
Posts: 14,112
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From: Florida
Ya gotta listen to the advice you got.
If you bench bled it, and then put the brass plugs in the line ports and the piston or pedal went to the floor, the mc is bad.
Could be the cup seals, piston assembly, or the bore in the casting itself.
If the mc is good you will get about 1/16" movement of the piston with the brass plugs installed and you will be able to stand on it hard enough to break or bend the back of your seat without any more piston or pedal movement.
Look down into the reservoir. You will see two sets of holes, each set has a large hole 1/8" dia fill hole, and then a smaller hole 1/32" dia compensating port in front of it. The seal sits in between the two.
With 1/16" of piston movement, the compensating hole is covered or passed by the seal and the brake fluid has no where else to go other than the line port that is plugged by the brass plug.
Hope that explains it for ya.
If you bench bled it, and then put the brass plugs in the line ports and the piston or pedal went to the floor, the mc is bad.
Could be the cup seals, piston assembly, or the bore in the casting itself.
If the mc is good you will get about 1/16" movement of the piston with the brass plugs installed and you will be able to stand on it hard enough to break or bend the back of your seat without any more piston or pedal movement.
Look down into the reservoir. You will see two sets of holes, each set has a large hole 1/8" dia fill hole, and then a smaller hole 1/32" dia compensating port in front of it. The seal sits in between the two.
With 1/16" of piston movement, the compensating hole is covered or passed by the seal and the brake fluid has no where else to go other than the line port that is plugged by the brass plug.
Hope that explains it for ya.
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From: Boca Raton Florida
My brake pedal goes to the floor. I have bleed the brakes too many times to count and can get half a pedal. As soon as I get the wheels back on and before I even drive it the pedal is on the floor again. Master clyinder remains full of fluid and there are no leaks anywhere. I think it is a bad master cylinder. It is a brand new chrome one from Tuff Stuff and yes I bench bleed it for a long time, no bubbles at all. I just unhooked the brake lines and pluged the port holes with brass plugs. When I depress the pedal it still goes to the floor. Has to be the master cylinder is bad. What do you guys think? I just ordered a rebuild kit for it and it is supposed to be here tomorrow. Is there any other way to test the master cylinder before I remove it.
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
I have the shallow hole. It must be the right one. I'll bleed it some more tomorrow, put the plugs back in, install on the car with the plugs in and see where the pedal goes. That should answer the question weather its good or bad.
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
Hey Noonie, That's pretty informative. I 've always been told to keep the m/c level when you bench bleed it but having the ports at 12 o clock makes since to me. I'll try bleeding that way tomorrow when it's cooler. It's 104 degrees in my shop now, way to hot to work on anything.
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
All right, I bench bled the m/c again, put the brass plugs in the ports and installed it on the car. Now when I press on the brake pedal it moves maybe a half inch and is hard as a rock. I assume that means the m/c is not bad. Does this also mean there is no air in the m/c? I will hook the brake lines up and bleed the system again when my buddy comes over. Do you think the reason for the pedal going to the floor in the original post was because there was air in the m/c.
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
Sounds like you now have the air out of the master. When you attach your lines,bleed each line a little at the master-leave the fitting a little loose-tap the line and watch the bubbles and fluid come out around the threads-have someone push the brake pedal lightly and more air will come out-tighten the fitting before they let up-do both lines-you should be good to finish bleeding.
I do think the reason the pedal went to the floor was you hadn't gotten all the air out.
I do think the reason the pedal went to the floor was you hadn't gotten all the air out.
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
(I've found if there's air in the 2 lines going down from the master,if you work slowly you can work the air up and out through the master fittings.)
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
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From: Hometown The best
Well, I removed the brass plugs, connected the brake lines to the m/c and bled the air out of the lines by looseing the flair nuts at the m/c while applying pressure with the pedal as Roger suggested. I bled the brake system several times while tapping the calipers with a rubber mallet making sure no air bubbles were coming out. I managed to get maybe a thrid of a pedal. Put the wheels back on, backed it out of the shop, put my foot on the brakes and the pedal went to the floor ....again!!!!! The brakes won't pump up, no leaks, m/c full of fluid. What the heck could be wrong. I might be getting a little flusterated !!!!
Race Director
Joined: May 2006
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From: Dayton, Ohio
Thread Starter
Racer
Joined: Mar 2005
Posts: 471
Likes: 1
From: Hometown The best
Yes , the m/c rebuild kit is here but I didn't pick it up because the m/c held pressure with the plugs in the ports. I thought the concensus was that the m/c was not bad as long as the pedal was hard with the plugs installed.