Thread: Brakes - bled completely & pedal is still soft?

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When I put in the new mc in my 74 Vega, I had the same issue - hard then soft pedal.

As usual it was my inept attempt at bench bleeding the mc. It took at least three attempts to persuade the bubbles out using the slow pedal down, fast up technique.

Good morning all!
First...Thank You! I have a lot to look into on this & have also checked the pedal ratio & come in at 3:1. So, along with the MC rebuild how do I go about changing the ratio? It was posted to move the shaft down approx 1.25" yes. But, Do you move the pedal assembly up to do this? wouldn't the MC shaft be at an angle if I just drilled a new hole in the pedal arm? I would love to post a pic of what I have here but, as of yet am not able to post pics.... any ideas on that? lol!

I think I've got the MC bled completely... no bubbles in the last few attempts. I was using the slow dwn/fast up tech each time I bled the lines also. It was an improvement for sure.

TGIF & have a great weekend!!

Sounds like your Master is bled fully. Regarding the pedal ratio, no you wouldn't move the whole assembly down. The ratio is the distance from the hinge/pivot of the pedal arm to the master cylinder push rod. I think before changing the ratio, you should get a good firm pedal feel first.

ok, so, next step is to rebuild the MC. The pedal height is 17(pivot to pedal) & 5.5 MC push rod to pivot.

Mack,
Like Mike says you need to concentrate on solving one problem at a time. Get your MC problem fixed, then you can assess your overall braking once they are working as designed. From the measurements you've given your pedal ratio now is 3.09/1 (17/5.5 - your 17" being from pivot center to brake pedal center where your foot engages). To change your ratio you'll have to move your whole MC assembly closer to your pivot point which means some fabrication work. Using your 1.25" example, you'd move the MC mounting point up 1.25", and then drill a new attachment hole in your pedal assembly at 4.25" from the pivot to keep the rod straight. This would give you a 4/1 ratio. Again, I'd try it with the MC problem fixed first, and be sure you still have a problem.

Just went back and re-read your post #1 and realized you're talking power assisted brakes here. Why would you consider changing your pedal ratio? I would think nominal 3 to one would be fine for power assist. Also, unless you already have a hone with good stones (and probably even if you do...) I'd strongly consider buying a new or rebuilt MC as opposed to re-building it yourself. I haven't put a kit in a MC for thirty years.

Hey Guys,
Ok, replaced the MC & have bled fully. The result is a "reasonable" pedal, still soft....about back to where I started. Roger, I went with a new MC, it was $23 - well worth the money! Thanks!
So, any thoughts on what to do next? The pedal ratio isn't something that would help?

If the pedal isn't firm now, it won't be firm after changing the ratio... did you look to see if you had those through the frame fittings?

They were known to trap air!

Good morning!
Ok, no through the frame fittings that I find, I was mistaken earlier.... it was a junction I was seeing. So, that is good news. God news on changing the pedal ratio also - wasn't certain how to accomplish that short of lowering the MC.
That brings up the question, how about the rod length on the booster? If it's too short that cause a low pedal yes? I'm getting a decent pedal at this point. it responds about 3 inches from the floor is all. I read that it is adjustable also... threaded...

Mack

A short rod length could be the source of all your issues. Someone mentioned to you to check for that.
You'll need to make sure the master cylinder is going through a full stroke.

34-40,
Thanks! You're right, It was mentioned early on to measure the stroke length. I blew it being focused on bleeding. I replaced the MC & booster now.... also replaced a section of brake line that looked wet. When bleeding I get a steady stream of fluid, no bubbles. MC bled fully, no bubbles. The pedal feels strong until you turn on the car & then the pedal drops to the floor. Advice?

Mack

SO the only difference is you bled this setup without the motor running? And then you ran the motor and the pedal drops/ dropped lower? Is that correct?

Try bleeding with the motor running if so.... doesn't sound like you got it air free with out the motor running.

Yes, that's the only difference. Ok, thanks 34-40!!

I've switched to a reverse bleeder, you service the car with brake fluid from the bleed ports to the master cylinder----no more problems, and it's a one man job....

Should not drop to the floor but with pressure applied to peddle, start engine the peddle will drop lower towards the floor, normal check any car


Instead of writing it out I found a link to paste that explains anything you would want to know about how a booster is tested or works.


To test See section 5 page 40

http://www.autoshop101.com/forms/brake05.pdf