New power disc brakes, soft brake pedal

[Reyals Bemus]

I just installed power disc brakes on my '64. I used Early Classic's conversion kit with dropped spindles. Its a great kit, very complete and very easy to install. It came with a new Raybestos 1 1/8" master cylinder. I'm also running a new MBM 11" booster listed for '67-72 trucks. Had to fab a threaded connector to hook it onto my existing push rod, but other than that it seems to be a nice booster.

Rear brakes remain completely stock. The whole system was in good working order before the conversion. Well... as much as any manual all-drum brake system could be.

The problem I'm having is a very light brake pedal. It seems to go down a couple inches before engaging the brakes. However, when it does, the brakes work really well. The truck comes right to a halt. But there is just no positive feel to the pedal! Before you guys suggest all the usual things, I'll just confirm what I know.

- I set it up for no slack in pushrod. Pedal immediately actuates the booster.
- Correct rod is between booster and m/c.
- Bench bled the m/c profusely.
- Bled the whole system repeatedly using the old school two man method. Open, push, close, release.
- Rear shoes are adjusted properly in the drums. No excessive play.
- No leaks throughout the system. Triple checked.

For testing purposes, I blocked off the m/c ports while it was in the truck. The pedal felt much higher and firmer, so I don't think my problem is in the m/c, but I could be wrong. Upon hooking it back up the rest of the system, it went back to feeling soft and low. Do I still have air somewhere? I've already poured two quarts of brake fluid through it in my bleeding process.

Any ideas?

[TopFuel1966]

Since you said that you installed the ECE kit I assume from that the front hoses are new. I would replace the rear hose as well. If it's the original hose it's coming up on its 50th birthday soon and it may have become soft and is expanding under pressure.

[Reyals Bemus]

Excellent thought TopFuel. The front hoses are indeed brand new. I'll order a rear hose and try it!

One other consideration I had was that the ECE kit came with two short 3/16" pre-bent lines that connect the m/c to the combo valve. The front circuit remains 3/16", but the rear circuit increases to 1/4" after the combo valve. Is it possible that this little 3/16" rear coupler line is causing some kind of issue with building pressure? Acting like a bottleneck or something? Feel free to tell me that it makes no difference.

[LostMy65]

I know you said you adjusted the pushrod, but for an experiment; try adjusting it out so it engages a little sooner.
I was having a similar issue, I adjusted the rod to engage sooner, and now my brakes are solid.
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[MikeN]

I totally agree with LostMy65. I just went through this...did not add disc brakes, but added a power booster to my previously manual four wheel drums. The pedal was way low, and soft at first after adding the booster. What I found is that very minor adjustments of the pushrod length make major changes in pedal height and firmness.

Think about it, due to the pedal being at the bottom of a long arc, away from the fulcrum at the top of the pedal assembly, adjusting as little as a 1/4 inch in pushrod length can change pedal travel by a couple of inches.

[Reyals Bemus]

Good points fellas. I will try lengthening the rod some. Right now its at zero slack. I can increase the length but it will push into the booster some. You don't think its a problem to be partially activating the booster all the time?

Just want to check.

Thanks guys.

[LostMy65]

Originally Posted by Reyals Bemus

Good points fellas. I will try lengthening the rod some. Right now its at zero slack. I can increase the length but it will push into the booster some. You don't think its a problem to be partially activating the booster all the time?

Just want to check.

Thanks guys.

You're not necessarily 'activating' the booster.

When you start your truck up, vacuum causes the booster to preload pressure against the Master Cylinder. That slack space is what your feeling in your peddle.
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[First c10]

I just replaced my brake booster and m/c. The PO had upgraded to a power booster from manual. Well he extended the pedal rod all the way out till less than 2 threads where holding the pedal adapter to the rod throw the firewall. Thank good the jam nut was tight. I tightened the rod to about 3/8" shorter so I would feel better about the rod being secure. But now the pedal is at the same night at the gas pedal. See picture. Do I need to drill another hole in the pedal arm or extend the rod from the pedal to the firewall pivot?
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[luvbowties]

Originally Posted by Reyals Bemus

I just installed power disc brakes on my '64. I used Early Classic's conversion kit with dropped spindles. Its a great kit, very complete and very easy to install. It came with a new Raybestos 1 1/8" master cylinder. I'm also running a new MBM 11" booster listed for '67-72 trucks. Had to fab a threaded connector to hook it onto my existing push rod, but other than that it seems to be a nice booster.

Rear brakes remain completely stock. The whole system was in good working order before the conversion. Well... as much as any manual all-drum brake system could be.

The problem I'm having is a very light brake pedal. It seems to go down a couple inches before engaging the brakes. However, when it does, the brakes work really well. The truck comes right to a halt. But there is just no positive feel to the pedal! Before you guys suggest all the usual things, I'll just confirm what I know.

- I set it up for no slack in pushrod. Pedal immediately actuates the booster.
- Correct rod is between booster and m/c.
- Bench bled the m/c profusely.
- Bled the whole system repeatedly using the old school two man method. Open, push, close, release.
- Rear shoes are adjusted properly in the drums. No excessive play.
- No leaks throughout the system. Triple checked.

For testing purposes, I blocked off the m/c ports while it was in the truck. The pedal felt much higher and firmer, so I don't think my problem is in the m/c, but I could be wrong. Upon hooking it back up the rest of the system, it went back to feeling soft and low. Do I still have air somewhere? I've already poured two quarts of brake fluid through it in my bleeding process.

Any ideas?

After transferring this scenario to a partner, he reminded me we had determined in the past that any time we add power boosters, with rods "properly adjusted", when we start the engine the booster "pulls" down a little giving us a little slack between pedal's upmost position and beginning-to-brake (or beginning-to-boost position). After completing these additions, we notice the pedal feeling higher and firmer while pumping it with engine off(like while bleeding)--feels kinda like manual brakes--altho' noticeably lower once engine is started.

We have noticed on later c10's with factory manual vs. factory power brakes, the factory power pedal often sits obviously lower than clutch pedal.

*In the end, the pedal-to-booster rod may be lengthened so as to take up some of the slack encountered only when engine is running. It should not, in effect, cause a pre-activated power boost or master cylinder pump.*

*This response is based on some actual experience while some is based on theory.*
sam

[ABX131]

What Lostmy65 said. I had the same problem with my disk brakes after I installed my new MC. It turns out the actual travel in the MC was longer than my pedal travel. Even though I had zero play in the pedal, It was not engaging the MC's piston at its most optimum point and needed to be lengthend about a 1/2" to really bite.

[Reyals Bemus]

Alright so I thought I would update this thread to let everyone know what solved my problem. It was a very simple fix after all.

I drilled a new hole 1" lower on the brake pedal lever. The pedal now feels perfect and the truck brakes incredibly well. The stock hole for manual brakes was apparently NOT providing the right leverage ratio to actuate the power brakes properly. All ailments are now cured using the new, lower hole.

Thanks to everyone who offered suggestions and other tips!

[luvbowties]

Originally Posted by Reyals Bemus

Alright so I thought I would update this thread to let everyone know what solved my problem. It was a very simple fix after all.

I drilled a new hole 1" lower on the brake pedal lever. The pedal now feels perfect and the truck brakes incredibly well. The stock hole for manual brakes was apparently NOT providing the right leverage ratio to actuate the power brakes properly. All ailments are now cured using the new, lower hole.

Thanks to everyone who offered suggestions and other tips!

Thanks for revealing to us that your problem was cured and what you did to cure it. So often we are left 'hanging'.
Sam

[ABX131]

By lowering the hole 1" you are basically creating more travel for the pushrod to activate the MC when the pedal is pressed. That is the same effect as lengthening the push rod. My concern would be the rod is now no longer being pushed in straight but rather at an angle which would cause uneven stress and wear on the receiving end of the piston. It should be straight in and evenly applied.

[luvbowties]

Originally Posted by ABX131

By lowering the hole 1" you are basically creating more travel for the pushrod to activate the MC when the pedal is pressed. That is the same effect as lengthening the push rod. My concern would be the rod is now no longer being pushed in straight but rather at an angle which would cause uneven stress and wear on the receiving end of the piston. It should be straight in and evenly applied.

Wherever the hole is drilled, since the brake pedal swings in an arc, the hole--and hence the end of the pushrod--is also swinging in an arc, constantly changing the angle from which the pushrod is being pushed. Since the piston in the m-cylinder (or the power booster) has an angular, or partially rounded, depression drilled as a pivot point for pushrod, the depression is very forgiving for this changing angle.

Also, the lower hole makes the pedal harder to push, as it loses leverage (of power) as it is moved farther from br-pedal's pivot point. Power brakes use this farthest hole, as power from driver's leg is now supplemented by the booster. Closer to its pivot point makes it easier to push, needed for manual brakes.

Wherever the hole is drilled, the pushrod is pushing straight inline on the booster or m-cyl piston only at 1 point(at only 1 of 360 possible degrees in the complete, tho' hypothetical, circle made by the arc) along the pedal's arc--earlier push could have rod angled down while later push has the rod angled up.

Regardless of the pushrod's angle, within reason, the piston will accept the push. Some models even come with more than 1 hole drilled in the br-pedal--the closer to pedal-pivot for manual and the farther for power brakes.

[Reyals Bemus]

Also, using the new hole actually aligns the rod better because I am using Captainfab's booster bracket which tilts the booster and m/c up from the firewall a few degrees. Its specifically designed to do this so that the pushrod hole can be lower while maintaing more or less a straight shot into the booster. Its perfect. I just didn't realize all this when I first put the system together!

[groundbound64]

Any chance you have a pic of how/where you drilled your new hole.. I'm going throught he same issue.. I'm going to adjust rod some more.. but if that doesnt work.. might have to drill new hole.

[LostMy65]

http://67-72chevytrucks.com/vboard/s...d.php?t=587057

.

[chevy_mike]

This is the biggest mistake most people make when upgrading to power boosters. You MUST have the the correct pedal ratio for the system to work correct. The hole where manual brake push rod connects to is too high and does not work correctly with power boosters. Power booster push rod needs to be 1" lower on the pedal. Many GM cars will have both holes (my '64 Chevy II wagon did). From what I have seen, it seems that GM did not do this for the truck line up.

Also, if you notice factory power brakes on '67 and later, the bracket that holds the booster/MC has a slight angle to it. This allows the rod to line up better with the lower hole in the pedal, to prevent binding. Problem is, '66 and older rarely had the PB factory option and many of the aftermarket kits do not account for this.

If you don't get the push rod onto a 1" lower hole, you will never get the right feel, too much pedal travel and can even be touchy to use.

[Guy67UK]

Sorry to revive an old post but just going through the pain of this myself..

when you drill the hole an inch lower is this from the centre line of the original hole or from the originals lower edge. I dont want to drill it in the wrong place as I would never get to drill the other one due to there proximatey.

[luvbowties]

Originally Posted by Guy67UK

Sorry to revive an old post but just going through the pain of this myself..

when you drill the hole an inch lower is this from the centre line of the original hole or from the originals lower edge. I dont want to drill it in the wrong place as I would never get to drill the other one due to there proximatey.

I've drilled all mine 1" center-to-center and had much joy from all.(Not all that critical!) Just FYI: I also tried to drill on the center of pedal-width.
Sam

[Guy67UK]

Thanks for the quick reply Sam

[cab4word67]

Love everyones post, I wound if you could shim the bottom of the booster to aid in the angle for the lower hole drilled?