Front wheels won't lock up under heavy braking.....

[Billhilly]

Hi Gents,

As the title says, long story but, it has come to my attention that the front’s aren’t locking up! And, don’t know if they ever did (!)

72 K2500, about 12k miles on all new brake components. 285/75/16 tires (33”).

Up front is stock booster, master cylinder, and calipers/rotors, no proportioning valve. Plumbed from the front of the MC, straight to the calipers.
Rear is Eldorado calipers on Chevy rotors, with a Wilwood proportioning valve.
EBC ‘Yellowstuff’ pads all round.

I can lock up the rears, but recently I played with the PV and only got worse results. I understand that by default the rears will be easier to lock up, less load etc. I don’t understand with all stock front components where the issue lies….


I started out with a 1” MC and it braked fine ish, but had a reeeeeally long pedal! Long enough to make your eyes go big wondering if it was actually going to stop! So I swapped to the stock (1 1/8th”) MC and got a ‘normal’ pedal. Thought things were ok……..

Any ideas as to where to start looking?! I’m all blank stares down here!

[geezer#99]

You need to hold a little pressure on the front pads. A 2# residual check valve will help.
Here’s an example.

https://www.amazon.com/Wilwood-Engin...AH1X1RZ0RDWR3N

A 10# check valve might also be needed for the back.

Plumb them into the brake lines close to the master cylinder.

[Keith Seymore]

You've also got a larger front tire on there than was stock.

The larger rolling radius increases the moment arm that the brake caliper is resisting, and increased width (and diameter) of the tire also provides a larger contact patch and therefore more tractive effort at the road surface.

K

[68Gold/white]

Does it stop properly, or are you just concerned that the front tires do not lock up???

[weq92f]

.
It would not hurt to run through the rotor seasoning and pad bedding procedures if both are new.

https://www.zeckhausen.com/catalog/i...Path=6446_6443

Basically what you do here is heat cycle the rotors to season them which helps prevent future warping and bed the pads into the rotors ( deposit evenly across the rotors a thin layer of pad material ) which increases the performance of the system.

You want to bring those rotors up to temp slowly at first, then hammer the crap out of them via hard braking without locking up the wheels. You want to see smoke and smell the burning of the pad material.

Hth,

-klb

[truck-kid]

if you do decide to install residual check valves, i would suggest a #2 in the rear since you have disc brakes there also. i believe 10# is for drum brakes

[RichardJ]

As per 68gold/white, locking up the wheels is not braking, it's just skidding. Locking the wheels just wears tires faster and increases braking distance. That's why they developed ABS.

Hydraulic brake pressure (psi) originates at your foot. Changing the 1" MC to a 1 1/8" MC requires greater pressure from your foot to achieve the same psi in the hydraulic system. As you noticed, that was accompanied with a decrease in pedal travel.

I would think a larger Booster would give you what you need with your disc/disc system. Then it's a matter of adjusting the Wilwood if the rears lock too easily with no load in the truck.

>>Up front is stock booster, master cylinder, and calipers/rotors, no proportioning valve.<<

The following drivel is in response to your need to include the "no proportion valve" on the front brakes. Just want to make sure the need for the prop valve is understood.

The purpose of a proportioning valve is to REDUCE the pressure (psi)proportionally.
What does that mean?
The two ports on the MC should always have the same pressure. A correctly installed proportioning valve should reduce the pressure to the rear brakes PROPORTIONALLY.
If you apply enough foot pressure to apply 1000 psi to the front brakes and 1000 psi to the proportion valve, the outlet from the proportion valve will reduce the pressure to maybe 750 psi to the rear brakes. 2000 psi from the MC, a proportion valve may reduce that to 1100 psi. At 400 psi, the proportion valve outlet may be 400 psi. No need to reduce rear pressure at lower pressures.
The parameters of any proportion valve are designed by the manufacturer of the vehicle.
An adjustable proportion valve(your Wilwood) allows you to alter the pressure reduction curve.

Very few applications exist that would require reducing front brake pressure with a proportion valve. Perhaps a 1500 lb show roadster with skinny front bike tires.



Residual Pressure Valves have NOTHING to do with brake pressure or the actual operation of the brakes. Residual, meaning after the fact.
Pressure in the brake lines after you use the brakes is only needed to keep air from entering drum wheel cylinders past the cups, but modern cup spreaders do that job quite well.
The exception and the need for stand alone Residual Pressure Valves is when the drum wheel cylinders and disc calipers are located under the floor board and lower than the MC. Gravity will pull fluid down from a wheel cylinder and suck air past the cups.

[HO455]

Sounds like it is time to do some brake pressure testing. Once you have some data then you can decide what your next step should be. There has been lots of good information posted but you need some data from your truck to know which way to go next.
This link is a basic description. There are lots of videos and such out there for you use, or take it to a qualified 4x4 shop if you don't have access to the equipment.
Good luck and please post your findings.


https://www.stu-offroad.com/suspensi...e-pressure.htm

[rpmerf]

If you are not able to push the pedal down all the way, you might be able to change the linkage on the pedal for more leverage.

[rpmerf]

Also, is that a RHD truck? If so, i'd be interested in seeing more pictures of it!

[truckster]

Originally Posted by Keith Seymore

You've also got a larger front tire on there than was stock.

The larger rolling radius increases the moment arm that the brake caliper is resisting, and increased width (and diameter) of the tire also provides a larger contact patch and therefore more tractive effort at the road surface.

K

This. It's a physics problem, and you've changed the variables. If the truck stops well I wouldn't worry about it. Locking them up won't stop you any faster.

[68Gold/white]

Originally Posted by truckster

This. It's a physics problem, and you've changed the variables. If the truck stops well I wouldn't worry about it. Locking them up won't stop you any faster.

I agree!!!

The guy on Fantomworks bothers me about how he test brakes. I'd hate to get my car back from him with 4 flat spotted tires!!!

[Billhilly]

Thanks for all your replies! Just to clarify, I understand that skidding is not braking, but I really don't have great braking! I haven't measured them, but visually the rear pads are worn more than the fronts. Not good.

Originally Posted by geezer#99

You need to hold a little pressure on the front pads. A 2# residual check valve will help.

A 10# check valve might also be needed for the back.

Plumb them into the brake lines close to the master cylinder.

Thanks, Dan, I'll look into them.

Originally Posted by Keith Seymore

You've also got a larger front tire on there than was stock.

The larger rolling radius increases the moment arm that the brake caliper is resisting, and increased width (and diameter) of the tire also provides a larger contact patch and therefore more tractive effort at the road surface.

K

Hey Keith,
This is what I was wondering. So the question then becomes how to increase braking pressure right?

Originally Posted by 68Gold/white

Does it stop properly, or are you just concerned that the front tires do not lock up???

No, it doesn't stop well, that is my concern.

Originally Posted by weq92f

.
It would not hurt to run through the rotor seasoning and pad bedding procedures if both are new.

Hth,

-klb

Thanks, and done on install to EBC spec.

Originally Posted by truck-kid

if you do decide to install residual check valves, i would suggest a #2 in the rear since you have disc brakes there also. i believe 10# is for drum brakes

Thanks, noted.

Originally Posted by RichardJ

As per 68gold/white, locking up the wheels is not braking, it's just skidding. Locking the wheels just wears tires faster and increases braking distance. That's why they developed ABS.

Yes, understand locked up is skidding not braking. I'm well shy of locking up

Hydraulic brake pressure (psi) originates at your foot. Changing the 1" MC to a 1 1/8" MC requires greater pressure from your foot to achieve the same psi in the hydraulic system. As you noticed, that was accompanied with a decrease in pedal travel.

Yup

I would think a larger Booster would give you what you need with your disc/disc system. Then it's a matter of adjusting the Wilwood if the rears lock too easily with no load in the truck.

Concerned to be 'the only one' who has to mod the booster

>>Up front is stock booster, master cylinder, and calipers/rotors, no proportioning valve.<<

The following drivel is in response to your need to include the "no proportion valve" on the front brakes. Just want to make sure the need for the prop valve is understood.

I didn't use enough proper words. What I was trying to say was I don't run a stock proportioning valve.

Originally Posted by HO455

Sounds like it is time to do some brake pressure testing. Once you have some data then you can decide what your next step should be. There has been lots of good information posted but you need some data from your truck to know which way to go next.
This link is a basic description. There are lots of videos and such out there for you use, or take it to a qualified 4x4 shop if you don't have access to the equipment.
Good luck and please post your findings.

Thanks, it would be real interesting to get some numbers... if I could find someone with the gauge set.....

Originally Posted by rpmerf

If you are not able to push the pedal down all the way, you might be able to change the linkage on the pedal for more leverage.

Good thought! I found an article on this exact thing in my searching for an answer. In my minds eye I thought there was two holes in the pedal arm. Nope, but alot of leverage with the stock pedal. Tricky mod but doable.

Originally Posted by rpmerf

Also, is that a RHD truck? If so, i'd be interested in seeing more pictures of it!

http://67-72chevytrucks.com/vboard/s...d.php?t=217490 Learn from my mistakes.... don't doeit!

Originally Posted by truckster

This. It's a physics problem, and you've changed the variables. If the truck stops well I wouldn't worry about it. Locking them up won't stop you any faster.

K, just need to improve the physics a little bit! It isn't stopping well....

Originally Posted by 68Gold/white

I agree!!!

The guy on Fantomworks bothers me about how he test brakes. I'd hate to get my car back from him with 4 flat spotted tires!!!

Certainly not aiming for that!

So if it's a simple physics equation (more bigger wheel/more grip etc) whats the solution?! The different pedal ratio is a option. (I don't really want to but) go back to the 1" MC and would residual valves shorten the stroke enough....? I'm not convinced they would. Bigger booster.... got a pretty big one on there now! (Which does raise the question, should/could I check the vacuum reading, and if so how, and what's the magic number?)

Thanks again!

[rpmerf]

In reading through this and the big question I am thinking: is your current setup malfunctioning, or does it need an upgrade? I am wonder how what you have compares to a JB7 braking system. Might poke around Rock Auto, look at some parts specs on MC size, caliper bore size, rotor diameter and width. I am more familiar with the C20 stuff, not sure how the K2500 stuff compares. I do remember in my 89 K5 blazer, the pedal having a long travel. My 97 suburban is the same way. Braking power seems fine, just a lot of travel, and no feedback.

On checking the booster -
With the truck off, push the pedal down and hold it. Start the truck. You should feel it drop.

Shut the truck down. Press the pedal and release a couple times. After about the third time it should get harder.

If in either of these situation, you do not feel the transition, there may be a booster issue.

Vacuum can play an issue if you have a giant cam. I think at least 16" is recommended. Measure manifold vacuum at idle.

A doc I found. Might be worth looking through to see if there is anything that hasn't been covered already.

https://www.classicperform.com/TechB...oubleshoot.htm

[57taskforce]

The best thing I’ve done for my K20 to date is adding hydraboost.

[weq92f]

.
One other thing comes to mind...I know you said the rotors are new but you might go ahead and mic them to ensure they are the proper width and within spec.

Long ago I did a brake job in a parts store parking lot ( something I did often ) and the guy turned my rotors too far by just a little bit. After finishing up I noticed the brakes just didn't seem to feel quite right ( much as you describe ) so I took the car to a brake shop and had them diagnose. First thing the guy did was mic my rotors and found them too skinny by just a bit under spec.


BTW there's lots of other things the about the Fantomworks guy that makes my skin crawl. His fundamental lack of understanding of testing brakes is an indication of other loose screws.

-klb

[RichardJ]

On post #1 the OP said.
>>about 12k miles on all new brake components<<

On post #13 you said.
>> but visually the rear pads are worn more than the fronts.

That sounds like the rear brakes have been doing all the braking. You said the braking was poor with the 1" MC and was little different with the 1 1/8" MC.
Two bad MC's? Bleeding problem?

You don't have a Pressure Differential Switch or a Combination Valve with a Pressure Differential Switch included?

[weq92f]

.
Do the rears lock up under heavy braking?

-klb

[Billhilly]

Thanks again for the input!

Originally Posted by rpmerf

In reading through this and the big question I am thinking: is your current setup malfunctioning, or does it need an upgrade? I am wonder how what you have compares to a JB7 braking system. Might poke around Rock Auto, look at some parts specs on MC size, caliper bore size, rotor diameter and width. I am more familiar with the C20 stuff, not sure how the K2500 stuff compares. I do remember in my 89 K5 blazer, the pedal having a long travel. My 97 suburban is the same way. Braking power seems fine, just a lot of travel, and no feedback.

Valid question, but I don't think malfunctioning. More likely needs an upgrade because of the larger tires. I am guessing I have JB5 = 7200 lbs. I would say exactly the same as C20 stuff? All 3/4 ton stuff the same?

On checking the booster -
With the truck off, push the pedal down and hold it. Start the truck. You should feel it drop.

Shut the truck down. Press the pedal and release a couple times. After about the third time it should get harder.

If in either of these situation, you do not feel the transition, there may be a booster issue.

Vacuum can play an issue if you have a giant cam. I think at least 16" is recommended. Measure manifold vacuum at idle.

A doc I found. Might be worth looking through to see if there is anything that hasn't been covered already.

https://www.classicperform.com/TechB...oubleshoot.htm

Thanks! I'll double check everything.

Originally Posted by 57taskforce

The best thing I’ve done for my K20 to date is adding hydraboost.

This is interesting......

Originally Posted by weq92f

.
One other thing comes to mind...I know you said the rotors are new but you might go ahead and mic them to ensure they are the proper width and within spec.

Thanks, I'll check just to make sure.

-klb

Originally Posted by RichardJ

On post #1 the OP said.
>>about 12k miles on all new brake components<<

On post #13 you said.
>> but visually the rear pads are worn more than the fronts.

That sounds like the rear brakes have been doing all the braking. You said the braking was poor with the 1" MC and was little different with the 1 1/8" MC.
Two bad MC's? Bleeding problem?

Yes, I would assume the rears are doing far to much of the work. 1" was fine (ish), long pedal was the issue. 1 1/8" shortened it right up, and felt ok. Really wonder if too much tire for the brakes.

You don't have a Pressure Differential Switch or a Combination Valve with a Pressure Differential Switch included?

No, but wondering if I should. Have just read recently that they automatically adjust the front/rear pressure as loading varies. Sounds like a feature I should have.

Originally Posted by weq92f

.
Do the rears lock up under heavy braking?

-klb

Yup, but if you try and dial it out with the prop valve braking....doesn't improve! (Less brakes).

So I can double check a few things, but, the question still remains as to what is the first 'change' (s) to make to start looking at improvements/eliminate things that aren't causing problems?

[rpmerf]

Poking around rock auto. Your master looks like a 1-1/4". They show a 1-1/8" and 1-1/4" for your truck.

Comparing to parts for a 72 K2500 to the JB6 and JB7 systems for an 83 K2500. The rotors look to be the same.

72
master 1-1/8 OR 1-1/4
calipers 2.9"

JB6
7,200 GVW
master 1-1/8"
calipers 2.9"

JB7
8,400 GVW
master 1-1/4"
calipers 3.2"

The pads on the JB7 look like they have a bit more meat to them also. Looking on SummitRacing, they show the same EBC Yellow pads for the 72 and the 83 (JB6).

Now I am really wondering how much those rear discs and prop valve are throwing everything off. On my Suburban (JB7), when the rear shoes are adjusted correctly, the pedal is much more responsive. When the rear shoes get out of adjustment, the pedal travel gets longer. The calipers you have are going to use more fluid than the stock wheel cylinders. I would suggest a stock prop valve, but I do not know there is a factory disc / disc prop valve. By the time they went to disc / disc, the proportioning should be done at the ABS pump. You mention automatic biasing, that usually requires a sensor at the rear to determine how compressed the suspension is.

I am wondering how much the brakes are bias to the rear. How are you determining how much to send to the rear? Going by lock up? You might be able to use a infrared thermometer to measure how hot each rim gets to determine brake bias. Even better if you can get the temp on the rotor itself. The hotter it is, the more braking it does. I'm really wondering if the front is handling enough of the braking, and if not, why?

[geezer#99]

Have you checked for actual movement of the cups on the front calipers?

[Billhilly]

Originally Posted by rpmerf

Poking around rock auto. Your master looks like a 1-1/4". They show a 1-1/8" and 1-1/4" for your truck.

Comparing to parts for a 72 K2500 to the JB6 and JB7 systems for an 83 K2500. The rotors look to be the same.

72
master 1-1/8 OR 1-1/4
calipers 2.9"

JB6
7,200 GVW
master 1-1/8"
calipers 2.9"

JB7
8,400 GVW
master 1-1/4"
calipers 3.2"

The pads on the JB7 look like they have a bit more meat to them also. Looking on SummitRacing, they show the same EBC Yellow pads for the 72 and the 83 (JB6).

Thanks for putting the time into this! I don't think I have heard of 1 1/4" in these trucks, but I've been wrong before.

Now I am really wondering how much those rear discs and prop valve are throwing everything off. On my Suburban (JB7), when the rear shoes are adjusted correctly, the pedal is much more responsive. When the rear shoes get out of adjustment, the pedal travel gets longer. The calipers you have are going to use more fluid than the stock wheel cylinders. I would suggest a stock prop valve, but I do not know there is a factory disc / disc prop valve. By the time they went to disc / disc, the proportioning should be done at the ABS pump. You mention automatic biasing, that usually requires a sensor at the rear to determine how compressed the suspension is.

Hadn't thought of the rears causing problems, they are the ones that work! A disc/disc prop valve is an option though. Here is a quote from the mpbrakes site -

The proportioning valve modulates pressure to the rear brakes so that as weight is transferred to the front wheels under heavy braking loads and pressure on the system is increasing, less and less pressure is being applied to the rear brakes. This minimizes rear wheels lockup as weight is reduced on the rear axle.

https://techtalk.mpbrakes.com/how-to...valve-overview

I am wondering how much the brakes are bias to the rear. How are you determining how much to send to the rear? Going by lock up? You might be able to use a infrared thermometer to measure how hot each rim gets to determine brake bias. Even better if you can get the temp on the rotor itself. The hotter it is, the more braking it does. I'm really wondering if the front is handling enough of the braking, and if not, why?

Yeah, very technical bias set up. Seat of the pants. Thermometer is a great idea. A week or so before I get on the road again, but I'll get the thermometer out!

Originally Posted by geezer#99

Have you checked for actual movement of the cups on the front calipers?

Nope, but I will. They definitely work, just not enough.

I will ring a couple of brakes places this week and see what they have to say. Thanks guys. I'll let you know how I get on.