Okay so I'll try to make this as short as possible but this has been driving me nuts for the last couple of weeks and I can't figure it out.

My brake pedal is soft when the car is running, by soft I mean there is no resistance what's so ever and the brakes do not work. I can't even pump up the brakes to make them work. The pedal is firm when the car is off.

I replaced the master cylinder, installed a new disc/disc portioning valve, all hard and soft lines except the hard line from the front to the back of the car. I have all new calipers and rotors everywhere. I switched to ls1 rear disc brakes from drum. I bench bled the master till it was bubble free and than I installed it. When I installed it, I bolted it in and had my friend push and hold the brake down. While it was down I threaded the hard lines to the proportioning valve.

I cracked the lines at the master cylinder to help bleed off any air trapped and then I bled the entire brake system. I used the pump method for 3 qts of fluid and it didn't help.

Then I tried a pressure bleeder which didn't work. Finally I used a vacuum pump and sucked 3 qts of fluid through and it didn't help either. I checked for vacuum leaks and there isn't any. I bought a booster and I will install it tonight.

If you have any tips or suggestions that would be great. I have been restoring this car for the last 8 years and this is the only thing stopping me from enjoying it finally.

I have searched numerous forums and they all seem to die off before there is an answer. The general consensus is that there is air in the line/master cylinder somewhere.

Thank you in advance.

 

Just figured I would update this thread if anyone in the future has this problem. My booster starter to squeak when the pedal was pushed, also the booster held vacuum and seemed to work. Well turns out the booster was the problem. Switched it out and bam they started working. So after making a pressure bleeder and buying a vacuum bleeder and bleeding almost 10 at a of fluid through it was the booster. You live and you learn

 

thanks for posting. I am actually about to work on mine. Kind of have the same noise and a squishing sound from the booster. But it holds a vacuum. gonna change that first now. the soft pedal made me think MC first. but I was dreading doing it. Here's hoping for the best.

 

Good luck! The nuts for the booster were 15mm and the steering column were 13mm if I remember correctly. Let us know if it fixed your problem. I have high hopes since the weather is finally breaking

 

You just convinced me to buy a new booster. My brakes have never felt good and I replaced and upgraded everything else. I put this in another post:


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New LS1 master
Earls braided lines
used LT1 booster
gutted prop valve for rear circuit
LS1 front brakes, new ceramic pads, new drilled and slotted rotors
PBR's in the back, new Drilled and slotted rotors
Rear axle hard line is new

Brake pedal is rock hard with car off.

I have buddy bled, gravity, and power bled with the Motive bleeder. Numerous times. New fresh fluid. No air in the lines. Bench bled the master off and on the car. Last attempt I spent an entire Saturday afternoon bleeding with 3 large bottles of brake fluid, Dot 4. I did get better pedal but still nothing before 2/3rds pedal travel.

Used the factory manual guide for bleeding lines at the master

Fronts will lock, rears will not. About 2/3rds of the pedal travel before I feel any significant braking force. From there to the floor it stops fine. But the low pedal is un-nerving.

********************************************************

Does this sound like your issue? The main reason I ask for clarification is that you stated you had no brakes and I have brakes but have to press the pedal 2/3rds of the way down to get them.

 

I am by no means a brake expert but I'll share my opinions.

I didn't have any brakes at all. I wonder if your brake issue has to do with the pressure output from the master cylinder as it is attached to the lt1 booster. I am curious if the amount of throw from the booster pin is far enough to engage the brakes. It's possible the brake pedal is low from the booster because pushing the pedal down is not a 1:1 ratio. Where you could push the pedal down and your not getting enough throw from the booster either because it's mismatched (I know others who used that set up without a problem) or the booster could be leaking around the diaphragm and not allowing full throw of the pin. As is takes more pedal movement to get the same effect. I hope that made sense.

Also I'm not sure about the gutted proportion valve but something doesn't seem right about it.

Also double check of all your pads are fully seated correctly I read a forum where he had similar problems and it was only a pad that wasn't seated well.

Good luck! Brake problems are annoying to figure out and the weather is finally breaking!

 

well, I changed the booster and the pedal felt exactly the same. it was harder with the car off but no change otherwise. so I changed the MC. took my time and did it right, then bled the CRAP out of the brakes. still the same. i gave up thinking I had air in the lines that I just couldn't get out and took the car across the street to a local shop. Picked it up late the next day after paying over the phone. Brakes felt the same. i took it back and told the guy "I know I asked for you guys to bleed the brakes but maybe I should have been more specific...I want you to bleed them until all the air is out of the lines and the pedal is firm so I can stop the car". When I picked it up later that day the car definitely stopped better, but the pedal was still going much lower than i felt it should. I jacked up the rear end to try to adjust the rear drum brakes but when I spun the wheels I can feel the shoes are already just barely making contact with the drums. so i don't really wanna adjust them anymore than that. It seems the engine I swapped has a more aggressive cam than I originally realized and the vaccuum on this engine is in the 10-12" range.
So, the car stops well when I push hard, I have a new booster and MC, and the lines are bled. Even thought the pedal goes a little further than I usually would like, I think I'm going to leave it as is.

What do you guys think?

 

Mjk have the rear brakes been replaced yet? Worn drums/shoes will cause that as will bad front brake hoses.

 

Thanks for the reply. I did change the rear shoes and drums last year. so those are fresh. But not any of the hoses. I will look into changing those and re-bleed the front brakes. I wasn't aware of that being a potential cause.
thanks!

 

Mine are a little different due to the cam when stopped at idle but not terribly so. I'm not sure if it helps the brakes but most have a vacuum reserve ball that helps maintain the vacuum.
Do the front brake hoses bulge out when you apply the brakes?

You're supposed to be able to adjust the rear brakes by backing up and aggressively hitting them. Sometimes you can hear them click etc. with the window open. Make sure it's safe to do so obviously.

 

Yes the stock rubber hoses can go bad and bulge, low vac will case a hard pedal, i ran 8" of vac on the street. As long as you pay attention its no problem

 

I have the same problem right now everything Has been replaced, the master cylinder, new front pads and rotors. I have braided stainless steel brake lines. One minute pedal is nice and firm. At stop light, the pedal goes all the way to the floor. I have to pump it in order to have a pedal.

 

It really sounds like you have an intermittently failing MC.

 

The master cylinder has just been replaced

 

? what parts are you guys ordering
i just priced a new mc & booster and everyone is asking $200 for reman units

what am i missing