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Discussion Starter #1
i posted a question a few weeks ago about my brake system, this is a 72 chevelle with drums all the way around. the master cylinder had fluid coming out the rear of it so i replaced it today. i read both the chiltons and haynes manuals on how to do this. they both have differnt answers of coarse!

one says to remove the clevis pin and disconnet the pushrod at the pedal. i did this although i have never done this before now. and reconnected it before i bleed the system.

one says there is a gasket behind the master c. i dont have a gasket just the rubber boot. ive never had a gasket do i need a gasket?

i did bench bleed the MC before it was installed. after i installed it i did a brake bleeding, but i used the one man bleeder, will this work on the non power brakes? i bleed them and saw bubbles come out so it was bleed until clear. after i did all tires i checked the fluid and topped it off. i never let the fluid get low or dry during the bleeding. but i noticed the pedal never got stiff ever when i was pumping it.
i started the car and the pedal goes to the floor to easily and no brake at all. i noticed that there is a clunk sound when the pedal is pushed to the floor and i hear a air sound coming from the pedal inside the car a hissing when the pedal is pressed. im pretty sure nothing was done inorrectly according to the guides but something isnt right for sure.

should i get someone to pump the pedal for me when bleeding?

i used the one man bleeder on my toyota yesterday and worked perfectly so i assumed it would work fine on the chevy but the toyota has power brakes.
 

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Discussion Starter #2
there are no leaks at any of the wheels or in the brake line at all. i checked the entire line yesterday and today after the bleeding and there are no leaks whatsoever.
i did a online search about the hissing sound and some people say its normal, some say its not normal. i didnt have a stiff pedal before now so wouldnt think the booster was bad. i im no brake expertl, i have changed brakes for years on my own vehicles with success just never encountered this before now.
 

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Are your brake shoes adjusted up to the drums? If not, you are going to get low pedal. That is probably the clunking you hear.

Get someone to help you bleed.......You can do it yourself, but you dont have to get up and down so much if someone helps..
 

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Discussion Starter #4
the brake shoes are adjusted out, i checked and adjusted them before i did the mc replacement so they are good to go. the clunk sound is felt in the pedal and is in the brake booster area. i know the brakes can make a sound when pressed like the springs moving, this isnt that sound.
this isnt a low pedal lol this is no pedal at all, even when i was bleeding the pedal never got tight feeling like it should in previous bleedings ive encountered.
i still wonder about the hissing sound at the pedal being a factor in this troubleshoot
 

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Wonder if you have a faulty rebuilt master cylinder.....I have heard this quite often with rebuilt ones as opposed to new ones.
 

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Discussion Starter #6
the mc i got is a remanufactured one, the one i replaced was about 2 years old it was also a remanufactured one. the pedal has always gone to the floor to stop in this car. i hate to take it to anyone around here to look at it as a brake bleed is 90 bux and to replace the mc with my parts is over 100 bux. i might have to fork out a few hunderd to get the brakes fixed after all.
 

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Discussion Starter #7 (Edited)
when i start the car the brake goes to the floor like nothing and i hear the hiss sound when pressing the brake pedal, not while the pedal is pressed all the way down just when pressing it do i hear this hiss sound. when i turn the car off the pedal builds some pressure after a few pumps but doesnt get stiff or hard and it still goes to the floor very very easily. the clunk sound to me sounds like the pedal is pushing against the booster, this sound is only when the pedal is pushed to the floor, maybe the mc is bottomed out? i mean there is no pedal at all goes to the floor.

i have read on other forums that if there is a hissing sound at the pedal the booster is leaking/bad and needs to be replaced even though the pedal isnt stiff like the classic case of a bad booster.
 

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customdrumfinishes,

The noise you are hearing is the brake booster hissing. This is normal so don't worry about this sound.
I think you have an air bubble in the system and you are going to need to "pressure" bleed the system. This is a tank that is under pressure and pushing brake fluid into your system all the time. You then go to each wheel and open the bleed valve until there is no air coming out. Do this to all the wheels starting at the right rear brake bleeder.
The one man bleeder works OK BUT you never see the fluid coming out of the wheel cylinder. There is a valve in the system that will block off fluid to either the front or rear brakes if the pressure between the two lines is unequal. So when you bleed the rear brakes you open the valve and then go step on the pedal. The pressure in the front brakes goes up but the rear system goes low because you have the wheel cylinder bleed valve open. That causes this slide valve to close the rear brakes fluid supply. You think it is bleeding the air but in fact you are just pumping the front brakes. Now you finish your bleeding of the rear brakes and you move to the front brakes. Now they are still working but the rears are not but you do not know that. You hook up your one man bleed tool and top off the master cylinder. You then pump the brake pedal. NOW the front brakes will have low pressure and the rear brakes get high pressure causing the slide valve to close off the front brakes. You never get rid of all the air.
See if you can rent a pressure bleeder from Auto Zone or Kragen to do this or get a few friends to help you out.

Good Luck
Scholman
 

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Discussion Starter #9
scholman, your explanation makes pefect since that the brakes are never properly bleed due to the valve, although i have bleed brakes in other vehicles with success, seems like the repair manuals would say to use a powerbleeder!

could the combination valve go bad?

the pressure bleeder you speak of, i have heard of this but never used one. im guessing i might could loan a tool one from autozone if they have one. maybe this explains the 90.00 cost of bleeding the brakes at any local auto mechanic.
i might replace the booster anyway due to it looks original and the fear it could go bad at anytime.
 

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russlaferrera
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Master cyl. bleeding

As a diagnostic check. . Can you make steel/brass plugs for the master cyl. then bleed it to see if you get a hard pedal? IMO you may have a faulty M/Cyl.....russl
 

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russlaferrera
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master cyl bleeding

If you disconnect the brake lines at the m/cyl and install plugs then bleed you will eliminate the rest of the brake system The pedal will do one of two things . It will be hard, which means the m/c is ok and there is air or something wrong w/ the system or it will not give a hard pedal. Which means the m/c is defective or something inside is not working right....russ
 
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