55.2-59 Braking Power

[57FleetsideC4]

Hi All, I finished my 57 pickup project this past summer (truck build in my sig.) and the one thing I do not like is the braking in I need more braking power. It has the stock corvette C4 brake calipers front and rear with a 1" master, 4 wheel disc proportioning valve, 8" dual diaphragm booster with 17" of vacuum to it. It stops straight and even but if I had to panic stop I would be in trouble. It has a good brake pedal feel. Anyone run into this? Thank you Steve

[nvrdone]

what is the dia of a stock c4 brake booster? If it is smaller than 1" you are loosing pressure. The brake components are designed to work as a package and when we try to mix & match, they dont provide great braking.

[leegreen]

Nice truck.
I see you had the calipers apart and rebuilt them, can we assume the pistons are not stuck and the slides are good? It is all stock c4 brakes? So mechanically the brakes are everything they should be?

Then I think that leaves a hydraulic problem.

How does the pedal feel with truck off and booster vacuum depleted?

What is the prop valve off and are you sure you have front and rear connected as designed?

is there a line lock or anything added?

[jwhotrod]

the problem is probably the vacuum booster. Ralph at ECI had told me that there wasn't anyway that a vacuum booster would allow you to generate the line pressure needed to run the corvette brakes. now you didnt say what engine you are using but a LS motor or anything with a high overlap cam wont generate enough vacuum to satisfy the brakes either. he suggested that i go to a hydroboost unit which gives the brake assist off the powersteering pump. this system is in the newer GM trucks from the factory. goodluck.

[dsraven]

have the new brake pades been burnished properly? if not then you will not get their full potential. also, what pads are on it now? some pads work well when really hot, like ceramic pads, but work less well when they are just driven around normally when they will not be running at high temps.
I assume there is no antilock brake system so I wonder if you are able to skid the tires on a dry road. if not, and all else is in order, then you likely have something out of whack hydraulically. on a gravel surface, at low speed, do all 4 wheels skid on a panic stop? (is your prop valve centered)

[dsraven]

I am also not a big fan of the small diameter vac boosters. I know they fit into small areas but a hydroboost, like mentioned above, is a much better system for consistent power boost, fits well in small areas, and can be sourced used for relatively cheap. they are also not that hard to mount if doing a swap. there are a few threads here showing how it has been done to our particular trucks, if you decide thats what your brake issue is

[57FleetsideC4]

Thanks for all the replies, This is a LS 6.0 with a 629 lift, 217in and 225 ex duration at .50. I have 17" of vacuum at idle which should be enough. I have noe tried stopping on gravel and it will not lock the wheels up on pavement. It does track straight when stopping. I have ceramic pads and slotted and drilled rotors and were broken in correctly like the manufacture wanted. Once spring gets here will have look into it further and maybe converting to hydraboost. Just trying to get some ideas to think about. Steve

[dsraven]

frame mounted booster or firewall?
if you disconnected the vacuum to the booster what your brake pedal effort need to be to generate the same braking power you have now? there are several variables that could have been inadvertently changed with the build but would affect the pedal feel and the amount of brake effort that could be applied with that pedal effort. one is the lengtg of the brake pedal arm. a shorter than stock corvette pedal arm and a different pushrod attachment point on that arm would affect the amount of force that could be applied compared to a stock corvette. master cylinder bore diameter, tire diameter compared to stock corvette and a prop valve that is set up for discs but does not specify the weight distribution of the vehicle. trucks are usually heavy on the front so more bias to the front may be what you need. a stop in gravel would indicate whether the rears are doing anything. a quick look at the rotors would tell if the rears are working but not whether they are getting any bias compared to front. it could be a simple as the prop valve wasn't centered after a brake bleed so one circuit is going without pressure. it would still stop straight, just not well.

[dsraven]

if all seems good maybe do a burnish on the pads again. when brakes are burnished pad material is "burned" onto the brake rotors to fill micro porosity and small defects on the rotor surfaces. this really helps with the friction material being able to grab the full surface available. at a shop where I worked a young mechanic did a brake job on a car and the owner came right back complaining of poor brakes, not being able to skid the tires like before. turns out the new mechanic didn't burnish the pads correctly. after burnishing the brakes worked well.
not sure if you know this, as most parts places will sell you ceramic if you are willing to pay the extra for them, but semi metallic pads actually have more biting power than the ceramic pads. ceramic pads also work better after they have been heated up some, the reason why they are not really recommended for very cold climates. semi metallic will leave more residue on the wheels though, and tend to not be as quiet as the ceramic pads. possibly try a set of semi metallic pads if you can't seem to get the ceramics to skid the tires, barring a different cause.
here is what i would do if it were my truck
check all possible mechanical reasons for less brake power. repair as required. ensuring all the components are sized and set up to work together, master cylinder bore size compared to displacement required, pedal mechanical advantage and travel of pushrod compared to master cylinder length, prop valve settings and whether it is centered, brake bias comparison, possibility that the booster is not operating correctly or possibly an electric vacuum pump is required to keep the vacuum at the level needed for the booster,hard brake lines are sized correctly, front and rear, and brake hoses are in good shape, etc. remember, just cuz a part is new it doesn't mean it is not defective
barring any mechanical issues get in and find a road that you can do some highway speeds but also be able to do some panic stops in safety. run up to speed and do a panic stop, like a really hard, quick stop. don't come to a complete stop though, but keep rolling. immediately do the same thing again. do this a couple of times until you see a small brake smoke puff coming from the wheels. then continue driving without having to stop, so the brakes can cool down well before stopping, where hot pads concetrated on a small area of the rotor will cause warped rotors and/or hardened hot spots which will become high spots on the rotor as the rotor wears. then retry the brakes after they have had a chance to cool off well. I like to pick a time of day when traffic is relatively calm and a road where i can legally do 60MPH for extended periods.

[57FleetsideC4]

Originally Posted by dsraven

frame mounted booster or firewall?
if you disconnected the vacuum to the booster what your brake pedal effort need to be to generate the same braking power you have now? there are several variables that could have been inadvertently changed with the build but would affect the pedal feel and the amount of brake effort that could be applied with that pedal effort. one is the lengtg of the brake pedal arm. a shorter than stock corvette pedal arm and a different pushrod attachment point on that arm would affect the amount of force that could be applied compared to a stock corvette. master cylinder bore diameter, tire diameter compared to stock corvette and a prop valve that is set up for discs but does not specify the weight distribution of the vehicle. trucks are usually heavy on the front so more bias to the front may be what you need. a stop in gravel would indicate whether the rears are doing anything. a quick look at the rotors would tell if the rears are working but not whether they are getting any bias compared to front. it could be a simple as the prop valve wasn't centered after a brake bleed so one circuit is going without pressure. it would still stop straight, just not well.

Thank you for the reply. It is a firewall mounted booster/master. The brake pads are hitting the rotors all the way around. The brake pedal mount was one made for this model truck and there is no adjustments on the pedal. Once I get it out in the spring I will try the gravel test. It has to be a basic thing just can't think it through. I have around 4-5 months to think about it. Steve

[dsraven]

do a little research on the best brake pad for your actual driving. just because youhave corvette brakes doesn't mean you are out doing frequent high speed stops as if you were racing. this comes down to really thinking about how you drive the truck normally. if you drive around town at low speeds or normal freeway driving then the super high performace racing pads would possibly not be the best choice as they would be used below their purpose under most driving conditions and would possibly not give the best results

[dsraven]

a brake pressure test at each wheel would be great information to have but not everybody has the equipment for that.

[hogfarm]

The brakes in my 54 Chevy worked great. I have disc up front and 9" ford drums in the back.As always I can't leave things alone. It took my truck apart and installed a 383 stroker motor ( just wanted more power and less fuel mileage).Things are OK but now my brakes take a lot more force to stop. I was thinking of a vacuum canister. I know nothing about a hydrovac system

[dsraven]

some suppliers say vacuum requirements for small dual diaphragm boosters should be up around 22 for best operation. possibly a vacuum pump would help with lower vac engines

[dsraven]

https://www.speedwaymotors.com/the-t...WBvM3np3--NykI

[mr48chev]

Locking up the brakes and going out of controls is not STOPPING POWER. Bringing the truck to a stop from a set speed without sliding the wheels in the shortest distance is STOPPING POWER.

On an AD truck if you lock up the brakes you are going to spin. Been there, done that.

From Road and track these are their tested C-4 stopping distances.

Tested performance figures by Road & Track magazine include a 0–60 mph (0–97 km/h) acceleration time of 4.9 seconds, a quarter mile time of 13.4 seconds, braking distance of 132 ft from 60 mph and 233 ft from 80 mph along with skidpad acceleration of 0.94 g. If you are way off from that 132 from 60 then you may need to make some adjustments. That is with 4 wheel disk.

The best thing is to get out in a big open area with clean smooth pavement and do some hard stops from certain speeds and see what happens and measure the distances and even compare them to the stopping distances of the donor rigs.

[mr48chev]

One more thought, if you are running Corvette suspension you are probably running tires that have twice or more the contact area that a set of say 215-75 15 tires might have and that will haul it down without loosing traction a lot better.

I'd go out to that safe place to play, Mark off a hit the brakes here spot and take the long tape measure and measure off some stops and see just how it does.

First is stopping straight without the rear locking up and causing it to want to come around. Second is stopping in a shortest distance and stopping straight.

I had a violent spin out in my 71 a few years back when a Geo Metro pulled out in front of me when the guy in the right hand lane made a turn on the road he was coming out of and I was hidden by that rig. It spun so hard that it broke the alternator bracket but I didn't hit and kill the guy in the little car and if I hadn't been paying attention he would be dead. The truck did a full 360 before it came to a stop. That was because my rear brakes totally locked up on bare dry pavement at 55 mph.

[57FleetsideC4]

Thanks Everyone, That is a lot of information to digest. I will check vacuum again but the pedal is not hard like no boost but could be wrong. I use the same pads as I do on my Oldsmobile "A" body cars and daily drivers and perform well. I guess I will get out there and test how much distance it takes to stop in the spring and go from there. It stops straight and smooth just don't feel like it has enough bite. I appreciate all the information and help. Steve

[e015475]

Between a 1" master cylinder, a cam and the 8" booster, you've kinda shot yourself in the foot by stacking up factors that decrease your braking.

A later model of the C4 would have had a 7/8" bore. By using a 1" master, you've increased your pedal force about 25% more than it would have been on the Corvette

Do you know the pedal ratio of the setup in your truck? If the ratio is too low it will contribute to the symptoms you've described. I'd make sure my pedal ratio was correct.


I'd start by making everything the same as the C4 - master diameter, pedal ratio and booster diameter.

If you don't get satisfactory braking with that, I'd put a Hydroboost on it. I've bought them out of junkyards and rebuilt them with a kit from Pirate Jacks and have had good results

[leegreen]

Factory c4 booster appears to be 9" single diaphragm, OP quotes a 8" dual diaphragm. If so he has more boost than stock. 30-40% more, cancelling out the loss of pressure from moving to larger bore master. In theory, if everything is working properly

current pedal ratio is worth knowing, as is stock C4, some math with ratio, booster diaphragm area and master sizes will let you compare to stock C4. There is some mystery around the servo valve inside the booster and how rapidly they let the boost come on with pedal travel, but short of dissection we will probably not be able to delve into that level of detail.

I'd test braking with booster unplugged from vacuum to make sure it actually works. As evidenced by the brakes getting worse when booster is unplugged, so be careful.

I still want to know what the proportioning valve is from, confirm it is plumbed right and if it incorporates a brake failure safety valve make sure it is properly centered (as DSRaven suggested earlier) Otherwise one end of the truck has no brakes

[e015475]

Originally Posted by leegreen

Factory c4 booster appears to be 9" single diaphragm, OP quotes a 8" dual diaphragm. If so he has more boost than stock. 30-40% more, cancelling out the loss of pressure from moving to larger bore master. In theory, if everything is working properly

current pedal ratio is worth knowing, as is stock C4, some math with ratio, booster diaphragm area and master sizes will let you compare to stock C4. There is some mystery around the servo valve inside the booster and how rapidly they let the boost come on with pedal travel, but short of dissection we will probably not be able to delve into that level of detail.

I'd test braking with booster unplugged from vacuum to make sure it actually works. As evidenced by the brakes getting worse when booster is unplugged, so be careful.

I still want to know what the proportioning valve is from, confirm it is plumbed right and if it incorporates a brake failure safety valve make sure it is properly centered (as DSRaven suggested earlier) Otherwise one end of the truck has no brakes

All great suggestions.

I understand the math of the force generated by a single 9" diaphragm versus versus the dual 8" should generate more force. I only mention it because when I was putting the brake system together for my truck, I heard anecdotal stories from a few guys who found them inadequate for the task, so I went with the Hydroboost under the floorboard (constrained by diameter under the truck)

The rear brakes on a pickup truck do about 20-30% of the braking so I'd focus on getting the front to lockup with pedal pressure in a range where I could modulate the brakes for the shortest stop. But like you, I'm curious what proportioning valve he's used, the plumbing and status of the safety spool/light.

You mention the rate at which the servo engages the booster, and I think this merits a little more discussion. I believe that there is a specific gap between the pin that sticks out of the booster and where it engages the plunger at the end of the master that has to be correct for things to work properly. With a mash-up of aftermarket booster and master cylinder, this might be off. Here's the tool to check it-
https://www.amazon.com/Upgraded-Adju...35179018&psc=1

Another thing to check is the vacuum check valve in the booster - a bad check valve can also produce the symptoms described

[dsraven]

with a modified engine, that may produce less than optimal vacuum to run a booster, some install a vacuum tank for added capacity. vacuum can go higher than idle when the vehicle is revving with the throttle plates closed (like down a hill holding back the vehicle, if possible with the trans thats coupled to the engine) so that added vacuum can be stored in the extra reservoir. shuttling the brakes can eat up any reserve vacuum pretty quick though so some muscle memory can help too.
I would suggest to go through the system you currently have and ensure everything is mechanically optimal, like adjusted correctly, sized correctly, the pedal advantage is correct, a full pedal application results in a bottomed out master cylinder with a little extra pedal travel as a safety factor, the prop valve is centered and operational, no twisted, flattened or tightly bent brake lines or hoses etc. then if it still doesn't give satisfactory brakes look into a better vacuum source or a different style of booster. I don't know anybody who hasn't totally loved their hydroboost conversion. if it comes to a booster problem

[57FleetsideC4]

Great information guys. I am going to check everything I have now to make sure operating correctly and go from there. I was so happy to get the truck on the road and going to shows and stuff that ignored this problem but knew I would have to address it. Thank You Steve

[jweb]

My setup is similar, 1" MC, 8" dual booster. I have Wilwood 11" front and Ford 11" rear rotors. It will stick your face to the windshield but won't skid. I fought several issues to get there.


1: My booster worked great but the little check valve on it was bad. It didn't allow air through either way. Just pull it off and blow on it, air should only go one way.

2: Pedal Ratio -This made a big difference and it's likely part of your problem. I have a firewall mounted booster/mc also and the "stock " setting wasn't right. Here's a good article about that https://mpbrakes.com/calculating-pedal-ratio/

3: Booster pin length - This is more if you have soft pedal but worth checking. https://mpbrakes.com/got-a-soft-peda...-cylinder-gap/

[dsraven]

JWEB, great job explaining quick and supplying those links. they help exlain several items that most home builders don't think about.
I remember those big tomatoe can vacuum cannisters used on stock vehicle back in the day. they worked but looked pretty cheap. lol. I suppose if a guy wanted to do a quick check to see if a vacuum reservoir cannister would help it would be easy enough to build a test cannister from ABS plumbing pipe and cable tie it to a fender or something. if it worked to solve a problem then something fancier could be made and placed out of sight but not far away. a check valve from a brake booster could be grommetted or glued in for the test res.