Improving pedal feel

[ULTM8Z]

I've done a lot of searching and reading on the subject and have posted in other forums, but figured I'd post here too given how may people have done 4-wheel disc conversions.

Basically I’m trying to improve the brake pedal feel on my 1971 Camaro. I have a Baer 4-wheel disc system that’s been on the car since roughly 2000. It runs good, but lately I’m wanting to improve the pedal feel.

The system came with the following:

C4 Vette calipers
15/16 bore MC
9” dual diaphragm booster (I believe it’s dual diaphragm based on memory of what I ordered).

I had also purchased the late 70’s WS6 Firebird prop valve and am not running the adjustable valve in the rear that came in the kit.

What I’m experiencing:

The pedal has a “late” feel to it (I’m pretty much half way to the max travel before significant brake grab is felt). My wife’s 2017 QX60 Infinity has the grab being felt in less than half that amount of travel. My 2006 Pontiac Vibe daily beater is not quite that good, but better than the Camaro.

Engine off to engine on, you call tell there is a horrendous amount of boost assist being applied, but again, the actual grab comes in quite late while driving. Although the car stops well, it seems like there could be some braking force being left on the table.

Doing a bunch of searching seems confirm my thinking, but I wanted to run this all by you.

According to my searching, the 92-96 Vettes had a 15/16” master cylinder as well, but a pedal ratio of 4.1. I have determined my pedal ratio to be 3.5:1. So right there, my brake pedal leverage is lacking compared to the Vette, which would tanslate into less braking force?

So I’m two things I’m surmising from this:

1. The booster allows me to get the maximum hydraulic pressure out the system, but the available braking force is less than what a C4 would get with the same MC and calipers.
2. The less-than pedal ratio for the given bore size is causing the brakes to engage late.

The 88-91 Vettes used a 7/8” MC and 3.5:1 pedal ratio. So going from 88-91 to 92-96, GM essentially traded a 1/16” on the bore diameter for 0.5 point on the pedal ratio, but maintained nearly the same final brake force at the calipers. Running the calculations with my pedal dimensions, it seems that 1/16” on the bore diameter is worth 3/4” of movement of the fulcrum.

If I’m correct in all this, then I need to increase the bore diameter to stiffen up the pedal.

I know I can always buy a cheapo MC to try it out before investing in a better one, but I’d like to see if I can get it right on the first try. Ultimately I'd like to get CPP's aluminum master cylinder.

So the million dollar question is (again assuming all of my assumptions and calcs are correct), would increasing the bore size to 1” provide a significant improvement? Or should I go all the way up to 1 1/8”?

[dontlifttoshift]

I would want 1 1/8" with a dual 9" booster.....at first glance.

Your lack of pedal ratio compared to a stock C4 should actually give a you a higher, firmer pedal than a stock C4.....the opposite of what you have. Have you checked the clearance between the booster pushrod and master cylinder?

http://techtalk.mpbrakes.com/how-to-...r-cylinder-gap

[ULTM8Z]

Hmmm....

Wouldn't an excessive clearance result in a significant amount of pedal play with the engine off? As it stands I get a rock hard pedal nearly as soon as I touch it when there is no vacuum applied.

The booster is the wild card in my mind compared to a stock C4. The only way to do apples to apples would be with no vacuum applied. If my booster is giving me a lot more assist than the C4 booster, couldn't it compensate for the pedal ratio deficiency when the engine is on in terms of pedal feel, but in reality the actual hydraulic pressure would still be lacking no matter how much assist I get?

[ULTM8Z]

Talked to Baer yesterday... they recommended going to with a 1" bore MC.

[morrisclassic]

Call Tobin @ Kore 3.
He is Great. Really helped with my brake setup.
13" C6 Front 13" Rotor 12" Rear GM
My experience is if you increase the bore diameter
the pedal is harder to push with less PSI delivered at the
caliper.
Can you drill another hole in the pedal to increase the ratio?
Billy

[ULTM8Z]

Yesterday afternoon, I pulled off the master cylinder and confirmed it's indeed a 15/16 bore. I also re-bled it and was surprised to see a few air bubbles coming out. I'm wondering if that may have been the problem.

Any rate, I need to buy a bottle of new brake fluid so I can re-bleed the system before I test it and see how it feels now.

More to come...

[Skip Fix]

So are you using the standard power brake hole/stud in the pedal? That was originally designs for the 11" single diaphragm booster 1 1/8" MC for most Disc/drum setups. The 79-81 Trans Ams that had 4 WD switched to a 9" dual diaphragm booster and 1 1/8 MC but also decreased the ratio on the pedal a little more as the dual diaphragm is more efficient. I hooks almost in the curve of the pedal. Some pictures over on nastyz28.com. When I first converted my 78 TA to the 81 4WD(has the aluminum quick take up MC vs cast iron used 79-80 for the 81 "low drag" front calipers) I kept the same pedal initially. Stopped fine but no pedal feel until I got the correct pedal. They are no longer available so you have to redrill for the lower ratio. If you need more ratio like a manual brake car you can swap to the upper hole often where the brake lightswitch has a bracket. But that will decrease fell having more ratio as you have more mechanical lever advantage.

[ULTM8Z]

My pedal ratio right now is 3.5:1. It's on the power brake hole (the other hole was for manual brakes).

I'm going to see how the re-bled master cylinder feels and go from there.

My setup apparently matches the 92-96 set up now, in terms of caliper piston to MC bore size. Only thing is I have a 3.5:1 pedal as opposed to the Vette's 4.0:1. So ostensibly I should have a firmer pedal. But the wild card is the booster at the moment.

Assuming nothing changes, I suspect I'm going to need the 1 1/8" bore (as opposed to Baer's recommendation for 1.00). This 9" dual diaphragm booster just seems a like A LOT of assist, so much so that it may actually overwhelm the 1" bore.

[Peter Mc Mahon]

Have you tried the manual hole just to see how it is?

[ULTM8Z]

The angle at which the pedal pushrod comes in precludes any other position.

[ULTM8Z]

I spent a good several hours trying to bleed the brakes but these bleeder screws on the calipers suck... they let so much air past the threads I can't tell if the brakes are properly bled. What's a good type of bleeder screw to get for these C4 calipers?

[ULTM8Z]

Hmmm these Russell speed bleeders may be the ticket...

[dontlifttoshift]

How much too short is the pushrod?

If the booster is not adjustable, I usually run a manual brake master cylinder and machine a slug or shim to drop in the deep hole and then adjust the length as needed to get the clearance I need.

The bleed screw should only be opened when the pedal is depressed, then it can not draw air into the caliper.

[ULTM8Z]

Looks like about .130" too short. Given my 3.5:1 pedal ratio, that's about a 1/2" of pedal travel that's doing nothing.

What I did temporarily (just so I can test the system with bleeding now a known-good condition) was cut the head off of a screw and file it down to .130 thick and the same diameter as the pushrod. I dropped it into the booster hole and then put the pushrod on top of it. I don't have a dedicated measurement tool for this, but near as I can tell I have between 0 and .020 clearance.

I got some Russell speed bleeders and bled the brakes last night. Holy smokes, what a difference in ease of bleeding... About 4-5 pedal presses per caliper and I had them all bled. Took all of about 15 min!

I plan to do a test drive around the block today to see how it feels. If there's marginal improvement, but mostly the same, then I'll probably step up to the 1" MC bore size (from my current 15/16).

[Skip Fix]

What booster is it? Aftermarket or a 79-91 TA for a second gen?

[ULTM8Z]

It's what came from Baer originally back in 2000-2001. It's a 9" dual diapghram.

[70chevellemalibu]

I am basically having the same issue, nice to hear someone who did a 4 wheel disc swap who isnt completely satisfied. I need to spend more time with my system but it sounds similar it brakes good but pedal travel is atleast half way through the movement before it starts to brake leaving me to believe there's lots of braking power left on the table.

In addition when i try to do a brake stand the engine power overcomes the front brakes....

good luck.

[ULTM8Z]

Question: Instead of lengthening the booster pin going to the master cylinder by ~.130, could I simply extend the brake pedal pushrod by that amount? My pedal pushrod is adjustable.

I'd effectively be somewhat preloading the booster diaphragm I guess, but I'm not sure how that would be detrimental to anything.

I'd simply start lengthening the pedal pushrod and watching the booster pin come out until it's the .130 further out that I want.


EDIT: just talked to Baer and they said no issues in doing this. Diaphragm in the booster should be able to absorb this "head start" on the pedal travel.

[dontlifttoshift]

Baer is wrong, don't do that. If youu preload the booster the control valve will be open all the time.





IF the bolt head you used as a shim is in place securely AND you are sure of your measurments, run it.

[ULTM8Z]

Good to know... thanks for the heads up