power 4 wl disc lacks power.

[68gmcJames]

New here , but hopefully folks can take pity on me and checkout my set up and see if they know how I should proceed. I've written the big brake manufacturers. Baer wrote back so far and is trying to help me out (which is awesome considering I'm using factory parts) but thought with a bigger group someone might know just what I need.

Truck is a 68 gmc

Brothers truck parts 8" dual booster with 1.125 bore old vette style master

Front is 11.85 x 1.25 rotor. D52 oem calipers

Rear is a cobra irs with stock brakes. 11.65 disc , 40mm caliper (internet said so)

I don't have a way to measure pedal pressure. Thought about analog bath scale...but I gotta find one. Max psi on master ports is 1400 , engine idling with 13-14 in of vac. ( I've read more vacuum would be better, but that's at idle vacuum , not built up from driving. Also I have a vacuum can.) Haven't checked at calipers yet.
That psi is pushing on pedal extremely hard. Same force without running on empty booster gives me 700psi. Brake lever was redrilled per instructions with brothers kit.

I have a jegs adjustable prop valve that completely open limited psi to 500. So I pulled the oring and reinstalled it to test. Rear pressure then matched front and improved stopping.

Front brakes have about 500 miles on them but about 15 years old and I don't even remember what kind of pads they are. No rust, stored inside. Rears all new stuff. Just stock replacement parts.

The truck stops decent . Probably better than it ever had really. But it doesn't have the power to lock up the brakes so I feel I'm missing out on some stopping power.

Tires are nitto 420s. 255/40/20 on 9" wheel and 285/40/20 on 10" wheel.

Lowered in front 4" with belltech spindles and springs. 1 3/8 swaybar. Adjustable rancho shocks.

Irs has qa1 single adjustable coilovers with 350lb springs. Stock cobra swaybar.

I have read through the stickied brake combo thread here and learned a good deal. It seems strange that the d52 calipers are supposed to create so much brake torque but I have this issue , unless the difference is in the pads.

I ordered some hawks hp plus pads for front and rear to try, but waiting on those and wanted to run this by everyone to get some thoughts.

Big bias to the front too it seems, but its a very front heavy truck I imagine.

Thanks for any help
James

[Apogee]

It sounds like you're on the right track with the HP Plus pads, although you probably could've just tried a $10 set of NAO's or semi-met's to see if it was indeed the current pads fault for the apparent low brake torque situation. That said, your vacuum is a bit on the low side, however the booster is providing a decent amount of gain (2X multiplier based on your rough estimates), so we can't really impugn your booster or vacuum levels.

Are you sure you had the prop valve completely open? A lot of people mistake completely open (all the way in CW) with the opposite where the valve becomes active immediately. Assuming the standard armature configuration in the APV, you should expect to see a 57% pressure drop across the valve, however that would mean that if it were active at 0 psi, then you would expect to see 700 psi up front and about 400 psi at the rear, not 500 psi.

As for the bias, the fronts should be locking up well before the rears given the effective piston areas of the calipers. I'm not even sure your application requires an APV since locking up the rears may not even be possible. Once the fronts lock and skid, weight transfer shifts weight back to the rear axle since your rate of deceleration is reduced.

Tobin
KORE3

[benno505]

Hydratech brake unit and never look back. best thing ever

[68gmcJames]

Thanks for the input Tobin. The apv was...weird. Its labeled backwards of how it (seemed to) worked. The knob says turn cw to increase, but gauge read zero all the way in. Ccw to decrease, but but that setting hit 500psi like a wall. I'm honestly not sure I follow your explanation of how the valve should function. When I took mine apart there was an serrated oring that seemed to block the in port so pulled it. I imagine you are probably right about maybe not needing one anyway though. Guess I'll just have to see how it does with the new pads. Maybe bandaid it with hps on front if it needs it? Or see if there is a caliper that I could use easily on rear. Kinda getting my cart before the horse though until I see how it works out now with current components and new pads.

I have read different pressure requirements quoted for disc brakes. I think my ssbc gauge says 1200-1400. Wilwood talks a bout excessive pressure as over 1200 psi. So, my current pressures "should" be fine correct? I considered going to a 1" bore master to bump it up a bit if needed.

I don't know how the manual brake guys get it done. All the charts I saw in the big stickied thread were quoting 500psi with 100lb pedal input. That would be a 250 lb or so pedal to hit my pressures.

Hydroboost would be nice...but ive replaced so many good parts on this truck already with something different that I really should have started on a new one. So trying to finish it up with minimum changes at this point. (Which means I'll probably want to redo it in a few years)

James

[FlyDoc]

Check your pedal ratio, that has an effect on perceived pedal input, and travel.

[bryant]

i had a similar result from a vacuum booster on an engine that idled at 15". had to put hydaboost on to get it to have power assist. you dont have enough vacuum to get any assist.

[68gmcJames]

I'll measure it to be sure. Truck was originally manual brakes. Kit had me redrill lever for power. Stock is supposed to be 6.5 for manual and 4.5 for vac boosted... my memory says its about 1.5-2" lower than original hole, so probably good... but good thought and I'm going to confirm.

And you got me thinking...

I'm half thinking about just getting rid of the booster. Engine compartment would look way better without it and the vac can. I keep checking my math thinking I must be wrong but it seems like pedal input would actually improve going from current 1.125 bore master with aprox 2x booster and 4.5-1 pedal switching to 7/8 master and 6.5-1 pedal.
909psi with 200 lb input (to simulate booster).99 bore area (1.125") 4.5 ratio
1083psi with 100 lb input with .60 bore area (7/8") and 6.5 ratio
That can't be right can it?
Also
1139 psi with 200 input (to aproximate booster) .79 bore area (1") 4.5 ratio

I found that stock they used a 1" with manual brake trucks with these front brakes. That would produce 822psi @ 100 lb.

Do these numbers look right? I drove this truck long ago with stock 4 wheel drums and manual brakes, but I can say without a doubt it stops with less pedal pressure now.

I'm new to all this technical brake info. I've been researching a lot, but I think I must be missing something, so sorry if I'm asking dumb questions. I know its making me feel dumb lol

[Apogee]

i had a similar result from a vacuum booster on an engine that idled at 15". had to put hydaboost on to get it to have power assist. you dont have enough vacuum to get any assist.

I disagree with that conclusion, mostly because many factory power brake applications from the late 60's and early 70's muscle cars didn't make 15 inches Hg vacuum at idle, some were significantly less. While more vacuum is generally better when it comes to vacuum accessories like boosters, it sounds like you may have had a bad booster. Low vacuum levels typically lead to low levels of boost and some pedal inconsistency. We like to see 14 inches Hg or more at idle and have rarely had issues at those levels. It's when you get below 14" Hg that we start to notice more pedal inconsistency and when you fall below 12" Hg, you start fighting an uphill battle IMHO.

The desired caliper pressures will vary depending on the caliper piston area, pad CoF, rotor diameter, etc, with the dominant variable really being the caliper piston area. Your D52 calipers have 6.7 square inches of piston area, and as such are designed to work at lower operating pressures than say a C5 Corvette caliper with 4.0 square inches. Ultimately, it's your tires that determine how much brake torque you can tolerate, so regardless of all of the various ways to get there, that's literally where the rubber meets the road.

The down side to just swapping over to a 7/8" bore manual master cylinder and 6-ish:1 pedal ratio with your current setup is pedal travel, which I suspect will be somewhat excessive. The 7/8" bore MC has approximately 61% of the piston area, so it's travel will be 39% longer than the 1.125" bore MC without changing the pedal ratio. If you change the pedal ratio by 44% as well, there could be travel issues.

Without more details, I'm just guessing, but I'd be surprised if you needed more than 700-800 psi to the front calipers to achieve lockup if everything is working properly.

Tobin
KORE3

[68gmcJames]

Thanks guys.

I have hard numbers now. Went and got a analog scale from walmarts today. I set it on my transmission jack adapter plate (knew it would be good for something) to keep it on a flat surface and put the yoke of it on the brake pedal. Put the pressure gauge in the rear caliper.

Tried it not running with a used up booster first:
50lbs got a tiny twitch
75lb got almost 300 psi
100 lbs got almost 400psi

Running (idle only after first test)
Was jumping up on gauge just getting everything lined up on the pedal.
50lb got 600psi
75 got 700psi
100 got 800
125 hit 950
150 hit 1050

I think I see the beauty of a booster now. Might only double psi at hard input but Huge increase at light input.

I guess it comes down to my marginal vacuum and maybe needing more agessive pads. Agree?

Baer said hydroboost. They thought a smaller master wouldn't supply neccisary volume for the big pistons.

[68gmcJames]

Measured pedal ratio original manual 5.33333 (12\2.25)
Current is 3.4 ....(11/3.25) low ratio right?
Measured hole to center of pedal. And hole to center of pivot shaft.
Need one in between I think but wouldn't want that many that close without welding the others up .

Pushrod seems to line right up with the hole I drilled too. But wouldn't be moving it much I guess.

[68gmcJames]

Doh! I need to fact check myself before I post.
I needed to use the total length. So 6.3 and 4.4. Matches what my research showed. Dang thought it was fixed.

[68gmcJames]

Sigh. Well, i found a few connections on the rear lines that i had just done that were wet so i tightened them up. So I took it for a short little trip just now. I don't know if there was some air in there that got out or maybe loosing pressure from those or I just wasn't confident in the system before all these tests and checks and conversation, so maybe I wasn't pressing the pedal as hard as I thought... But it stops good. Not a ton of assist like my new truck , but easy modulation and after a few decent stops I mashed on em pretty good and they locked a little. Repeated a few times to see if it was a fluke. Pretty easy to keep it right on the threshhold. Engine stalled on the last hard stop. Wish I had my data logger going. Think might have lost fuel. I have a sumped drag race cell with external electric pump. So that might be my next thing to sort out.
So thanks for the help and ideas on this guys. Especially all the tech stuff Tobin. Really appreciate it. I'll let y'all know how the hawk pads do. Hopefully get me down to a lesser pedal input for max braking. Not horrible as is but surely over 100lbs for lock up right now.

James

[Apogee]

Glad it's getting sorted out...FWIW, lockup at or around 60-80 pounds of pedal effort is generally very driveable, with good modulation up to that point. I would be surprised if the HP Plus pads didn't get you there, but without knowing the current pad CoF, only testing will tell.

[68gmcJames]

Hawk pads showed up today! Got em on and got garage cleaned up ready to take the truck out and started raining. Tomorrow. I hope.

James

[bryant]

the car that i had the booster problem had a cpp kit on it. their booster and every thing. cpp's product could very well have been the problem. in the end the hydraboost was the fix for that car. i was scratching my head for months trying to get that car to stop. cpps excellent tech help (bs) was very happy keep telling me to adjust the parking brake until i finally got pressure gauges on the calipers and told them the pressures then they said it needed hydraboost.
regardless, the knowledge on this board is far higher than cpp seems to have. i will always go with korbins advice and recomendations.

[68gmcJames]

Hawk pads got me there! Might be a little too much actually(but rather too much than not enough). Pads I had before must have been horrible. Break squeal isn't anywhere near what I was expecting from internet posts either regarding the HP plus pads. Only heard 2 quiet little squeaks out of about 50 stops on the ride I just took. Trucks kinda loud though too. Might hear more if I stuck them on the kids Honda or something.
68 feels like it stops harder than my ram which I thought had great brakes. That thing has a Huge booster and looks like 14" rotors. No issues with stalling again either and fuel cells pretty low now. Shrug.

James

[Skip Fix]

My 78 TA has the factory 81 TA 4WD dual diaphragm booster and has worked fine for various motors down to about 9" at idle. The factory also even used a reduced ratio pedal compared to the single 11" booster disc/drum setup. 15" as Tobin said should not be an issue.

The factory 79-81 TA 4 WD used the smaller dual booster 1.125" MC D52 front calipers and a 2.5" rear caliper similar to the D154/metric caliper. If that type booster and sized MC was used for that system and a 3900 lb TA your system is close except the smaller piston rear calipers. They worked OK except the rear ratchet E brake setup.