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| Username | Post: Brake pedal geometry -w- new booster & master cyl? No pedal after bleeding. | |
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1956chevy Gold Supporting Member Posts 286 
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 04-26-12 02:35 PM - Post#2219410
Hello all. I have got my new 7" dual diaphragm brake booster, master cyl and proportioning valve installed. I have all new brake lines with 1/4" to the back, and 3/16" to the front (2 lines). I installed check valve bleeders at all four wheels. It is a disc/disc system. Here is the unit I installed.   When I rebuilt the pedal assembly I drilled a new brake hole an inch lower than the stock hole, as I had read a lot about using the lower hole for newer boosters. I confirmed with the seller of the brake parts that I should use the new, lower hole on the brake pedal. When I tried to install the pushrod and clevis it is way too low to line up. I could get it installed if I tried, but the angle of the pushrod into the booster was about 30 to 40 degrees. That can not be good for the booster. I called the seller and they said to use the stock (upper) hole so the pushrod would be straight into the booster. That is what I did. I bench bleed the master cyl very carefully, then installed it after having no bubbles in it. I bleed all wheels using the check valve bleeders I installed, and clear tubing into a small bottle that had a magnet on it so I could hang it above the caliper each time. All four wheels have no air in the lines, I have no leaks, but I barely have any brake pedal. Does this have anything to do with the reduced distance the pushrod is traveling? I guess what I am asking is, what is the purpose of the lower hole in the brake pedal for the power booster? If anyone uses this, how do you get the brake pushrod to line up? That side load on the booster can't be good. Any ideas?
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roccaas Contributor Posts 647 
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04-26-12 03:11 PM - Post#2219418
I'm going through this on my '63. Mine is a stiffness when you first push the pedal, and then a hard pedal with no give at the bottom of the stroke. Apparently, you have to keep the same pushrod length when changing MC/PB's, and there are many sizes out there. I'm going back to my original MC and have it rebuilt.
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acardon "9th Year" Silver Supporting Member Posts 9470 
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04-26-12 03:34 PM - Post#2219434
Usually the angle of the bracket that mounts the booster determines where the hole should be in the pedal. It looks like the bracket you have is straight so the original hole would be correct. You will loose a little motion but I don't think it would be enough to cause a problem. .... Link tp brake pedal. I have a booster mounted flat to the firewall and use the original hole and it works fine. Is the pedal to booster rod the correct length? Is there too much clearance between the booster pushrod and master cylinder? See "test freeplay" in above link and .... Link to booster pushrod.
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Rick_L Honored Member Posts 24809 |
04-26-12 04:18 PM - Post#2219447
Don's comments are on the money. I would add this. When you drill that hole lower on the pedal arm you are reducing the travel, but you are also decreasing the line pressure for a given force on the pedal. A 7" booste, even a dual one, doesn't have a lot of boost, so with those you don't want to change the pedal ratio and lose line pressure. The other reason for mounting the booster at an angle is to make it clear other things in the engine compartment but the 7" boosters usually don't have problems because they are small. If you use the stock pedal ratio, you should still have sufficient travel if you didn't have a booster, and adding the booster does not change any of that. What I would do if I were you would be to first check your rear brake adjuastment. This affects pedal height with discs and power just like in the stock system. The other thing is if you have rear drums you need residual pressure valves somewhere in the rear lines. If you used a GM type disc/drum combination valve it has one, otherwise you may need to add it. It will raise the pedal a bit. After that it's a matter of getting the air out of the lines, calipers, and wheel cylinders. |
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tewitt1949 Contributor Posts 252 
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04-26-12 07:37 PM - Post#2219526
I bought a master cylinder and booster simular to yours but I'm running drums all the way around. I'm still trying to touble shoot my problem so I'm not really sure whats going on. My problem is I think I am running out of stroke on my MC. If I adjust the shoes up quit tight they seem to work good but if they loosen up any the pedal seem to bottom out on stroke. The pedal goes about 3/4 of the way to the floor then its hard and stops. I've go it rod mounted in the new hole I drilled. Any ideas? One thing I forgot to mention is if the MC cover is off, when I push on the brake, I see a fluid squirting upward in front resivor. To me not that I'm giving it more thought it sounds like the rod isn't adjusted correctly. Your thoughts please. When the brakes work, they work great and are very sensative. |
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Rick_L Honored Member Posts 24809 |
04-26-12 08:19 PM - Post#2219548
Air bubbles squirting up through the reservoir is pretty normal especially with long strokes. But only on the release. |
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tewitt1949 Contributor Posts 252
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04-27-12 05:16 AM - Post#2219605
Moved to a new posting. |
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1956chevy Gold Supporting Member Posts 286
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04-27-12 06:23 AM - Post#2219622
Thanks you all for your responses. Don, thanks for the link from CPP. That is great info. In retrospect it seemed that while bleeding the fluid only came out during the last half of pedal travel. I do not have any extra play in the brake pushrod into the booster, and I am not pre-loading it. The booster nipple into the m/c was supposedly set prior to shipping it. I never checked it. It did seem recessed more than it needed to be. I have no time to work on it until next week. I will have to start over and remove the m/c. I will check to see if the booster nipple is adjusted correctly or not. I will update the forum then.  The brakes don't work, but it sure is pretty!! Thanks everyone! 
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4dr 57 "6th Year" Silver Supporting Member Posts 2762 
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04-27-12 06:57 PM - Post#2219809
Yepper, it is pretty..dam pretty! Niice  Just in case it was not covered..you can gauge the distance between the MC and the Booster using a piece of clay. The distance between the pedal to the back of the booster should be the same as the MC to booster -very little play. The booster has an adjusting screw for this adjustment. It's easy. Takes some time, but it's a can of corn to do. I raised the back of my car once when bleeding the rear shoes and jiggled the rubber hose in case of an air bubble there but that seemed dumb because the fluid probably will not "slide past" an air bubble, will it? ...dam good looking I just hope a 7" booster is enough for your ride. I needed an 8" to get any real stopping power. Stanley
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1956chevy Gold Supporting Member Posts 286
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04-28-12 05:34 PM - Post#2220049
Stanley, that is exactly what I plan on doing. I will remove the m/c and use clay to check the booster adjustment. It seemed like the booster pushrod was recessed more than it needed to be when I was mounting it, but I never checked it since the seller specifically told me it was adjusted prior to shipping.  I noticed that CPP does recommend tapping the calipers while bleeding to knock loose small air bubbles stuck against the sides of the housing. Perhaps you were on the right track wiggling your rubber hoses. :-) I am going to start another topic on 7" boosters. I am curious how they work.
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tewitt1949 Contributor Posts 252
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04-28-12 07:16 PM - Post#2220082
check out this link which was given to me on the other site since I'm also having trouble. Good info. The part that interested me was the part that says power brakes have to have a minimum 18 oz of vaccum. I got 10oz. |
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