Another master cyl shot! (help me PLEASE!)

[Duner97]

I bought my truck with the brakes not working, I bought a new master cyclinder. worked at first, then shot. I bought another one, worked a little bit, brakes needed bled, so as i was bleeding them, BAM, lost all pedal, and killed another master cylinder. These dang brakes are the only thing preventing one more classic chevy from being on the road. Any suggestions guys?

[Stout72]

Are you bench bleeding the master cylinder before installing it?

[SWINGSAX68]

did you check the rod adjustment (I doubt thats it but check it any way)
Check your wheel cylinders for leaks, does the master maintain fluid???? Check everywhere for leaks, make sure you replace old rubber lines. it sounds like air in the system.
Bench blead the master.....or you can do it on the truck. You will need two to tango. break loose the fitting to the master, you or freind pump 3 times hold on 4.........then you or freind loosen the fittings into the master cylinder one at atime just enough to allow air to escape and use a towel around them so it don't spray everywhere. do that a few time and check the fluid and pedal occassianally. Now do the same thing with the two brake lines that go into the proportionaing valve...break em loose. pump, grab towel, open em up...do this a few times. I have a feeling that the master cylinder was not the problem. sometime you need to bleed like this for them to bleed properly.
It could also be that the brakes are not adjusted properly, this will lead to low pedal as well....both these problems occured when I replaced my master. I hadn't adjusted the brakes in a long time though.

[Duner97]

I replaced both wheel cylinders when i replaced the MC the first time.
I bench bled the MC both times i replaced it and bled the lines coming out of the MC. I did have pedal after replacing it, but during the bleeding process, i lose it. (MC goes out)
I can pull the MC off the booster and use a screwdriver to push the piston in the MC to pump fluid. Fluid is coming out the back of the MC, so that is how i know its bad.
a bad proportioning valve wouldn't mess up the MC, would it?

Autozone isn't going to like me, but i like the lifetime warrenty! I'd like it more if it all worked right.

[SWINGSAX68]

maybe not sure.....but if you have a compressor, why dont you take the prop valve off and remove the sensor and blow it all out. worst thing that could happen is you forget which way the sensor goes in LOL. well thats if you got drum/drum. What is your brake set up. youve got a power booster I know that. I don't what else to suggest.

[rage'nrat638]

GO BUY THE GOOD STUFF AT NAPA..........


RETURN THE AUTO ZONE ONE IN THE NAPA BOX TO NAPA....
JUST GET A REFUND.......


DOHOOOO

[Blue_71]

i would venture to say "auto zone" is the problem here

[gonebad2]

GM still sells master cylinders for these trucks. I would stay away from the rebuilds. The bore is often pitted and or over bored. I doubt something that is bought at Autozone would be sleeved to repair the bore.

[Castiron9]

Get a professional brake mechanic to do the job..Then if there is a problem "they" can fix it............

[BLACK AND BLUE 67-72]

Check your rod, it could be too long and that could be what is causing this, it sounds like it might be over travelling and killing the rear seal.

[Duner97]

I've got discs up front, and rear drum. I didn't get a rebuilt, it just seemed too cheap i went ahead and got a new MC.
I'm about ready to go with the professionals!
As for the rod, that is what was in the back of my mind. I messed with the adjustment. before i messed with it, it seemed as if that rod wasn't doing anything until my pedal was past the halfway point. so i adjusted the rod out more. That could be the problem.
I measured it out and adjusted the rod so that so that as soon as i give it pedal, the rod is pushing. Should there be more space in there? What is the proper length for the push rod?

[tom hand]

The brake pedal should have freeplay.....it should move about 1/2 inch before it contacts the MC...if it doesn't the MC will not return all the way and will be be impossible to bleed.

[BLACK AND BLUE 67-72]

Adjust the pedal so there is 1/8" of pedal free play. This is what the service manual says to do. That is your problem you must have a small amount of free travel or it can ruin the master cylinder, or make it impossible to bleed as tom stated above.

[Duner97]

I'll have to go exchange for another MC and do some adjusting i guess, then see what happens. You guys said 1/8" or 1/2" of free play. do you know how much of a gap that is between the tip of the pushrod, to where it makes contact in the MC?

[SWINGSAX68]

check that rod buddy. I would suggest getting a manual rod and bleeding that way and then putting on the booster....just a suggestion. But you need to have at least 1/4-1/2 inch of play or like was mentioned above, the master will not releive itself, causing problems. If you start experiencing low pedal, soft pedal hard pedal stuff then this is your problem. It happened to me.

[rage'nrat638]

if you put the master cly on with the lines off........

and when you tighten it down..you see brake fluid start to piss out the line holes....that rod is out to far to start with....

take a pencil stick it in the rear hole on the master cyl...
and make a mark flush with the rear of the master....

so you know who deep the hole is......

then ....making sure the rod is pushed all the way into the booster.....place the pencil next to the rod and make it a 1/8 shorter than the mark on the pencil ....now were getting somewhere....

install the unit...and check for pedal freeplay......

[Duner97]

rage'nrat... That is pretty much what i did the last time, maybe i just didn't leave enough of a gap, maybe i misjuged how far back the MC mounts onto the booster, or maybe the rod wasn't all the way in the booster. thats assuming i did mess up (me... very possible) I'll mess with it some tomorrow if i get time. i'l have to wait on a new one though.