May have finally sorted my less than stellar brake performance...

[FLYNAVY30]

Posting this separate from my build thread as it will hopefully help someone else in the future.

My truck is a 1972 C20 Suburban. A few years back, I completely gutted the brake system. It was originally a factory disc/drum set up and the only part that remains is the original pedal itself. I'm using Wilwood D52 calipers with the stock discs on the front, with Wilwood's D52 rear calipers and the rear disc from a square body K20 and a custom bracket from a company that is now out of business. I'm running a Baer 1 1/8 master cylinder and a replacement stock 11" brake booster.

Since I put the system together, it has only worked "OK". The truck stops perfectly straight, but it's always seemed like for any given braking scenario, I needed slightly more force on the pedal than I should have. I recently contact Wilwood about switching to a more aggressive pad compound, and they said that would work, but it would also result in faster pad and rotor wear. The tech made a point of including that they recommend a minimum of 18 in. Hg at idle, and an 8 in. DUAL DIAPHRAGM BOOSTER....

I immediately went out to my truck....showing 15 in. Hg at idle, so not ideal. Then I started doing some research on single vs dual diaphragm boosters. The math to figure out how much "boost" the system is applying is:

(vacuum at idle) x (booster diameter) x (number of diaphragms)

So based on Wilwood's recommendation of 18x8x2, you're looking for 288 "units of boost?"....not sure what that boils down to as far as units. I then did some research, and realized that the '72 trucks came with a SINGLE DIAPHRAGM booster because with the disc/drum set up, you dont want/need as much boost. So running the math on my truck as it sits... 15x11x1 = 165 "units of boost"....obviously well below Wilwood's minimum recommendation for a power brake system. On doing some more research, I quickly realized the '73 trucks, which had 4 wheel disc brakes, came standard with an 11 inch, DUAL DIAPHRAGM booster..... part #2232NB at Summit for $145.99.

So even with my lower vacuum at idle: 15x11x2 puts me at 330 "units of boost" which will hopefully alleviate my lackluster braking performance. I'll report back as soon as I get the new booster in!

-Greg

[Mike C]

How well do your brakes work at speed when you jump off the throttle and mash the brakes? That deceleration with motor should spike the vacuum and let you know if more booster push would help.

Try it in a 3rd gear or even 2nd same test to see if high vacuum situations make the brakes feel better.

I have manual stock disc setup in my '69 1/2 ton, and it stops HARD. On my '70 3/4 ton I converted to later model discs and kept the smaller diameter drum brake booster, and it worked well but you could tell it had a cam in it since it did stop better in high vacuum deceleration scenarios when getting on the brakes.

[FLYNAVY30]

Stops better in hi vacuum scenarios as you described, but still requires a boot load of pedal. I got the '73 and later booster mounted today....required some modifications to the bracket, but nothing crazy. Now I'm waiting on an adapter for the pedal push rod because the rod in the booster is 3/8-16 and every aftermarket push rod is 3/8-24....nothings ever easy! Hopefully the dual diaphragm booster will net some improve braking. It would make sense since the '73 and later trucks came with 4 wheel disc brakes.

[pwdcougar]

Wow let us know how it goes!

Paul

[Mike C]

I don't think 4 wheel discs were available on a truck until the GMT900 trucks. '99 and up. And I know they were not on the square body trucks.

I am very interested in hearing how it works for you. Always looking for a relatively inexpensive upgrade!

[FLYNAVY30]

Originally Posted by Mike C

I don't think 4 wheel discs were available on a truck until the GMT900 trucks. '99 and up. And I know they were not on the square body trucks.

I am very interested in hearing how it works for you. Always looking for a relatively inexpensive upgrade!

Hmmmm....interesting. I didn't do a lot of research beyond the fact that the factory brake booster for '73 and later was a dual diaphragm while '72 and earlier was a single diaphragm. I just assumed it was because you don't want as much boost with the drums in the rear.

I've got the whole thing installed, just waiting for these adapters to show up for the pushrod.

[Bigdav160]

I was under the impression that all C/K 20's had dual diaphragm boosters even the 71-72 models

[FLYNAVY30]

Originally Posted by Bigdav160

I was under the impression that all C/K 20's had dual diaphragm boosters even the 71-72 models

I couldn't tell you for certain, but I do know that the brake booster I previously had installed was an 11 inch, single diaphragm booster as that is what comes up as the factory replacement for a '72 C20 at all of the major suppliers (Auto zone, Advanced, Oriley's, Rock Auto, Summit Racing, etc.) The dual diaphragm booster comes up as a factory replacement for the '73 and later trucks.

The mounting brackets on the dual diaphragm booster are not the same, which would point to it not being an OE application, but again, I honestly don't know. It will be interesting to see if there is any braking performance gained with the swap. For $145, I was willing to roll the dice.

[Mike C]

I'm going spend some time trying to figure out what GM did and when with boosters. I'm sure the early squares were single diaphragm. I have a couple of C2500 trucks both have vacuum boosters and I have a couple of 3500 a C30 and a K30 and they both have hydroboost. The 3/4 ton trucks are an '83 and an '85 and the boosters DO look thicker than the 71-72 trucks so will be interesting to see what I can find.

[FLYNAVY30]

Ill be curious to see what you come up with. I wish I could provide you with performance results at least, but I'm still waiting on parts. Hope to have a road test by Friday afternoon though.

[Mike C]

I started by checking out Rockauto. I pulled up '74 C2500 and '86 C2500 trucks. They share NO booster part numbers between the two which was interesting.

The '86 trucks listed both a 3 1/2" thick booster which must be the single diaphragm and a 6 1/2" which I assume is the dual diaphragm. Not sure what years these changes happened, but clearly sometime between the early 70's and mid '80's.

Will be watching for your road test on the new booster.

[randy500]

Those 2 boosters are the same diameter and both dual diaphragm.
You are changing pedal ratio by removing the stock pedal linkage. Im not sure if you gaining or loosing ratio.

[FLYNAVY30]

Originally Posted by randy500

You are changing pedal ratio by removing the stock pedal linkage. Im not sure if you gaining or loosing ratio.

I don't think that is correct. Pedal ratio is defined as the Distance from the pedal pivot point to the pad surface, divided by the distance from the pivot point to the push rod attachment point. Neither of those dimensions has changed by changing the booster.

[randy500]

Originally Posted by FLYNAVY30

I don't think that is correct. Pedal ratio is defined as the Distance from the pedal pivot point to the pad surface, divided by the distance from the pivot point to the push rod attachment point. Neither of those dimensions has changed by changing the booster.

What about the firewall linkage on the original booster? Or does that linkage thta your removing not effect pedal ratio?

[FLYNAVY30]

Originally Posted by randy500

What about the firewall linkage on the original booster? Or does that linkage thta your removing not effect pedal ratio?

To the best of my knowledge, that additional linkage does not affect the pedal ratio. The pedal itself is essentially a lever. The relationship between the two measurements outlined above gives you the “ratio” which is essentially how much that lever is multiplying the anount of force you are applying to the pedal. That linkage on the other side of the firewall isnt amplifying the applied force so much as redirecting the same force in a different plane....ie higher up the firewall. I suspect, this was due to the need to clear the valve covers on the trucks equipped with the big block engine.

If Im off base on any of this, someone please jump in. I make no claims to be an expert.....Im also learning as I go.
Posted via Mobile Device

[jjzepplin]

I will follow along from the sidelines.

[FLYNAVY30]

Got everything buttoned up this morning and went for a quick drive around the neighborhood. Initial impressions:

- the pedal is sitting higher now...although I may shorten the length of the push rod to pull it back towards the firewall a bit

-the pedal feels softer, has more travel, but the brakes feel "crisper" and "grabbier"

-I dont know that brake performance has been improved any, but the amount of pedal force required for a given amount of breaking is definitely decreased.


I need to drive the truck around in day to day traffic, in addition to getting it up to speed and really jumping on the brakes hard before I can give a final verdict but initial impressions are that it was worth the effort if I can get the pedal height down some.

[FLYNAVY30]

Can someone measure the distance from the back side of their stock pedal to the floor/firewall for me if you get a chance?

Thanks!

[FLYNAVY30]

Did some more driving today and confirmed that the booster swap was worth the effort. Some key takeaways:

- total breaking performance has NOT changed any. 100% max effort panic stop still results in just barely locking up the front tires

- pedal feel is significantly improved. The amount of force applied to the pedal for a given amount of breaking has significantly decreased

- pedal feel is "softer" but effort is more proportional to that of a modern car

At this point I am 90% happy with the brake system on the truck. I have ordered a set of E-compound Wilwood pads for the D52 calipers to replace the BP10 pads that came with the calipers. The E-compound pads have a higher coefficient of friction across the temperature range which will result in more dust and faster pad wear, but on a truck that sees about 6K miles per year, that's a trade off I'm willing to live with for better initial bite and more friction across the board.

The big takeaway from this whole experiment is that vacuum matters. My LS motor pulls 15 inches at idle....according to Wilwood, you really need 18 for a boosted system, and at this point, I certainly believe them. The booster and pads are really band-aid solutions, making up for a lack of vacuum.

Eventually, when I pull the body off the frame to fix the rust, I may revert to a manual brake system, retaining the same calipers, but swapping in a Wilwood 7:1 ratio pedal and a 15/16 bore master cylinder. Manual systems are much less forgiving of mis-matched components, but when you get it right, the system really does work well.

In the end, I'm calling the booster swap a success. Let me know if anyone needs specific part numbers. I'll update this thread when I get the new pads swapped in later in the week.

[HO455]

Originally Posted by FLYNAVY30

Can someone measure the distance from the back side of their stock pedal to the floor/firewall for me if you get a chance?

Thanks!

Measuring from the firewall at 90 degrees to the back of the pedal arm. I read 6 15/16".

[HO455]

It sounds like you have made good progress. And thank you for the informative posts.
The importance of good vacuum is an issue I'm well acquainted with from past white knuckle adventures.
I have had good luck with Hawk pads in the past if your still not satisfied.

https://www.hawkperformance.com/hp-plus

[04ls1gto]

I installed the CPP hydorboost on mine and its like tossing a anchor out the window. I would highly recommend moving to this setup. It really drives like a new truck

[FLYNAVY30]

Originally Posted by HO455

Measuring from the firewall at 90 degrees to the back of the pedal arm. I read 6 15/16".

Much appreciated! I was able to get the pedal back to the stock position by adjusting the rod length, and then the brake light switch to match.

[FLYNAVY30]

Originally Posted by 04ls1gto

I installed the CPP hydorboost on mine and its like tossing a anchor out the window. I would highly recommend moving to this setup. It really drives like a new truck

I had considered it, but I think if I were to go that far, I'd probably choose to go with a purely manual set up and install a correct ratio pedal over the hydroboost. I honestly prefer the feel of a manual brake set up.

[Bigdav160]

I went hydroboost also. Love it.