New master cylinder, brake pedal on floor next morning??

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dgibby

dgibby

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Before I put the car away for the winter I noticed a very low brake pedal. Bled them and it help for a day or two then almost to the floor. Last weekend took a good look and saw fluid running down the firewall so I replaced the master with a reman unit. Bench bled it, bled all 4 wheels and had a nice solid firm pedal. Woke up the next day and noticed my brake lights were on and figured I hadn't adjusted the swith right. Nope.. got in and the pedal was all the way on the floor. Pumped it and got pressure but not as good as before, will check tonight and see it's on the again.
Any ideas??
Manual brakes. Factor front disc, rear drum. No leaks, calipers overhauled and lines replaced last year, rear shoes and wheel cylinders replaced year before. Rear shoes properly adjusted.
 
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what kind of new master cylinder? aluminum or cast iron? Where did you get it?
 
I had 1/2 dozen reman'd master cylinders fail, practically during the bench bleed process ( or so it seemed ). Luckily they were from the local part counter where I could take back all most as fast as it failed.
I did recently buy a master cylinder online, Rockauto or Amazon I forget. It is/was a new one though. So far so good. Good luck with yours.
 
Think I figured it out....

I'm no expert but I'm pretty sure this shouldn't be coming out the back of the master when you look up under their dash??

IMG_3348.JPG
 
No. There should be a tear drop shaped tab screwed to the back to keep that from coming out.
 
Only 1 pizzy little sheet metal finger for a keeper ( various shapes with reman'd ). Two screw holes in the casting though. Might be incase one strips out, or screw rusts into the casing, breaks off?
I've never seen 2 fingers installed.
My guess, their keeper wasn't stronger than the spring.
On a positive note... this might be repairable and usable still. Pull the master cylinder. Reinstall the piston with a good keeper... or 2. More brake fluid.
And one more note.. If when you put the bolt back through the rod at pedal... Don't pull upward/outward against the keeper when tightening that bolt/nut.
At minimum that can damage the brake lamp switch or alter its adjustment. At worst? well...
 
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Yeah the little teardrop thing was bent over and the piston was sticking out the back. Not going to bother to try to reassemble it, will replace it with a quality unit.
 
I believe that tear dropped piece of sheet metal is for shipping purposes only. I'd bet that many have been put together wrong with just that tab. There should be a plate on the back of the master with a boot on the back of it (which is why there are two holes). Mine got bent up when pulling up on the pedal to get the push rod out (stuck retaining o ring). Made a new one out of 1/8 aluminum and some 1/4-20 bolts. I wonder how many of these were put back together with just that tear drop. Scary thinking just that little tab holding it it. Power brake MC should have a snap ring holding in the piston. Lots have been done to these vehicles over the years with people of various levels of experience working on them and how we find them doesn't always mean it is right.
 
I've got the rubber dust boot. Never seen a retaining ring for this boot. Someone please post a pic of that.
 
They came that way for years with the small tab and worked. You can not pull up on the pedal or you will pull the piston out. The cast M/C are fine to use. After all they worked on many cars for many years before the aluminum ones were made.
Dumb *** tow truck drivers will use a seat belt to tie the steering wheel in a fixed position so they can keep the front wheels straight. They would tie the belt from the brake pedal to the steering wheel and tighten it up and you guessed it, pull the piston right out of the master cylinder on manual brake cars.
 
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