The Stovebolt Site Forums The Shop Area Driveline not happy with disc conv. 3/4 ton

Is anyone not completely satisfied with their disc brake conversion on their 3/4 ton? Did mine back in 2015 and I'm not saying inadequate but just not up to what I expected. The calipers, pads and rotors came from a 1982 Chevy 4wd 1/2 ton. The bracketry was made to fit perfectly. I used a CCP dual m/c, proportioning valve and an 8 " booster with a vacuum reservoir. They stop well enough at 45 mph but at 65 I don't know if I would rear end someone. The pedal is great, goes down just a bit when you start the engine like all boosted systems do. No matter how hard I jam on the pedal I can't lock up the brakes. Which I know decreases your stopping distance. You want to be just at that critical moment before lock up to get the best stopping power and shortest distance. I learned this in Motorcycle safety school. The rear brakes I've kept adjusted. The engine vacuum is 18" here in Arizona at 3000 ft. That should be plenty. I wonder if switching to an organic pad instead of a semi-metallic would be beneficial? This is one of the first kits I found for the 3/4 ton. Plenty for the 1/2 ton. They put disc brakes on everything nowadays so they should work as expected. Please feel free to give me your opinions good and bad please.
Sharon

I am happy with my 48 3600. It sounds like I have bigger calipers and rotors though. I did a back axle swap, used a 2004 chevy 2500 axle with disc brakes. In the front I have the Hollister Road (now out of business) disc brake kit. It uses rotors and calipers from a 2003 chevy HD2500. I have a m/c from a 1991 chevy 3500 with 4 wheel discs.

Will yours lock up the back brakes? It would take some research, maybe trial and error, to see if you need a better match between calipers and m/c. The bore of the m/c plays a big part in break pedal force to hydraulic pressure (aka caliper strength).

I have had a similar issue before with junk pads. It was on a unrelated platform but it was terrible. I would stand on the brakes and it felt like brake fade. They worked but nothing I felt “confident” stopping fast with. I swapped pads that were probably some off brand to a regular wagner or raybestos and i might have cut the rotors and it was pretty much normal after that. Had me confused for a while. On my 3/4 ton I keep the original drums adjusted up and if I hit that pedal you are going into the windshield. It will 100% lockup the brakes at 55mph. I drive it carefully just because of what it is but I cruise on the highway with it at 65-68 and have no problems on off ramps besides a little pulsation thats only really noticeable above 60mph stops. I have no idea what brand shoes are on it but they are from 70’s or earlier. They looked fine so I ran them. It does have all new hydraulic parts front to back including a dual master.

ASU

I'll check the rotor diameter this morning. I have 16" wheels and the caliper just clears the inside of the wheel. Don't think I can use anything bigger. I"ll adjust up the rear shoes this morning, its been at least a year. How can you determine the m/c size that will work best with the calipers you have? Does the brake line size to the fronts have an effect on pressure? I used new lines that were the original size on the truck when I rebuilt it. On another note, I talked to the guys at CCP and they said I could try removing the o ring at the rear of the proportioning valve to get more pressure to the rears. Don't know if that is a good idea because I have shoes back there. I can"t lock up the rear brakes.

Sharon

You will have to inspect the MC, perhaps there is a casting number on it you can read? Otherwise have to pul the pushrod, remove the rubber bellows, measure the ID with calipers to get the correct size! Perhaps you have paperwork from the initial installation that would indicate MC size?

Ed

Do I need a larger diameter bore on the m/c?

A smaller diameter master cylinder bore would help the situation. The pedal stroke will increase somewhat, but the hydraulic advantage between the master cylinder and the caliper will improve. That sounds counter-intuitive, but it works.
Jerry

Thanks Jerry, I think its a 1"bore but I will check. Maybe go to a 7/8 or 3/4 " bore.

When I put disk brakes on my 39, I went to a wrecking yard and pulled the hard parts off the same pickup that had disk front, shoes in the back. I bought a new M/C, new shoes (the all new front brakes were part of my heidts kit). I did not have any issues with it stopping even from 80-90 mph on the freeway.

Actually I found the receipt and its a 1 1/8 bore m/c. I found a 1" bore for a 76 corvette without power brakes. Thought I might experiment with this one. I do have a question on the line routing to the proportioning valve. Does the line from the end of the m/c go to the front of the valve where there are two exit holes? One is blocked off. And the one where the pushrod enters go to the rear of the valve where there is just one exit hole and is also the largest? This is how the pic looks on CCP website for the m/c with booster. I've ran it this way for 2 years now. Seems like a stupid question I know, but you look on their website at another m/c for my truck and the lines do not cross over one another from the m/c to the proportioning valve.

Sharon

First of all we don't know what year your truck is. That would be nice to know. Anyway here are some facts and questions.

1. A 1982 K10 uses a 1 1/8 bore MC. So that is what you want.

2. We don't know if your MC is on the firewall or under the truck. This makes a difference.

3. Your Proportioning valve may be faulty or the system needs residual valves....or a possible bleeding.

4. Your complaint/issue is hard to decipher. It stops at 45, but you don't know if it stops good enough to not rear end somebody? What does that really mean? Check the stopping distance and compare to a chart. Also "unable to lock the brakes"? Brakes should lock up and vehicle should skid. This is why ABS was invented. What makes you think brakes are not locking up? Surely the rears are, because you keep them adjusted? So if the fronts don't lock, it is a proportioning/residual issue. The proportioning valve is mostly for rear brakes. It's basic function is to make the rear shoes grab a little before the front, so you don't nose dive and....... the shoes have farther to travel and need a head start over the front pads. The front pads MUST BE KEPT very close to the rotor. That is what a 2lb residual valve in the front line is for. If the fluid tries to drain back to MC, It allows pads to get too far away. That is NO GOOD. A proportioning valve may not prevent this. That's why I asked where MC is mounted. Where is the proportioning valve mounted?

5. A residual valve keeps the pad from retracting too far. There are no return springs like on the shoes. So pads are supposed to just "relax" a tiny bit due to slight loss of pressure and probably the spinning rotor force.

Sharon, see what you think about all this.

My truck is a 54 3600. The m/c is mounted on the frame where the original one was. The proportioning valve is mounted below it. I don't have to pump the brakes so I don't believe a residual valve is needed. The rear tires never lock up because you see no rubber on the road. Nor any rubber from the front tires. I guess I'll have to go walk off 100 yards and see what the stopping distance is at different speeds. I was just looking for some help, sorry to ruffle your feathers.
Sharon

This shows how the "proportioning valve" should be plumbed. It's really a combination valve as it has the metering valve, brake warning light switch and proportioning valve all in one.

https://www.carolinaclassictrucks.com/78-87-GM-FS-Truck-Prop-Valve.html

Did not ruffle any of my feathers. You are looking for help. I had some questions, advice and observations. I'm probably not gentle enough for sensitive people. I just want to get enough info as quickly as I can so as to help as quickly as I can. If I have a question, I ask it, and explain why I am asking. There are as many approaches as there are people in this world.
Stay safe and good luck.

Since you can't get any of the tires to make black marks on the pavement, I suspect that your master cylinder push rod is not adjusted properly or set up properly, or the new master cylinder is defective.
Half of the master cylinders which I have bought in recent years have been defective. (Maybe only 1 out of four, but it seems like way too many, nevertheless.)

Bartamos, I've had a real crummy day and responded poorly. I take your advice to heart and will try to inform readers better.
Sharon

Just looked at GordandFran post, looks like I've rand the lines to the wrong ports on the proportioning valve. I swear I did it according to CCP's diagram. I'll switch them in the morning and see if that makes a difference. I've always wondered about that. Thanks

Awesome news about the plumbing, glad you checked it.
Thank you for your note. I understand my approach was too direct. Hope you have a better day. Your brake problem is important to me.

Couldn't wait until tomorrow. There are 3 different thread sizes on the m/c and the valve. Looks they will mount up only one way with the front m/c line going to the rear inlet of the valve and the rear m/c line going to the front inlet of the valve. Still up in the air.
Good Night

OK. What has changed in the last 2 years? Is the booster still getting vacuum?
https://www.stovebolt.com/ubbthread...-lack-of-stopping-power.html#Post1242948

I would like To reiterate; the residual valves are definitely needed for low mounted MC/PV. I think these need to be installed. The proportioning valve does not have a residual feature.

Yes there is vacuum to the booster. The brakes were never very good after the change over, as I would think they should have improved. I wonder if I should drop down to a 1" bore on the m/c to get more pressure. I really worked at getting the pushrod depth correct on the m/c also.

I did forget to mention that the original CCP m/c started leaking at the boot so I replaced it with a new one from Autozone. Brakes performance never changed. That was 2 years ago.

Do I put a 2 psi in both the front and rear lines?

Just how far does the pedal move when fully standing on the brakes? How much clearance from pedal to floor at full travel ( standing on pedal )?
Have you really tried a panic stop in a parking lot? 35 mph stand on brakes panic stop? 40 mph? 50 mph? Everyone should to know how the truck is going to act!
What is the surface of the rotors like, polished smoothed, slightly polished, dull, grooved?
How about the pads, worn some, worn none, real dusty, slightly dusty?
How many stops can you make in a short span before running out of vacuum? Or does it always have plenty of vacuum on the booster?
Are you 100% sure the master cylinder is hooked up ( plumbed ) correctly? front and rear in correct locations?

Pedal travel is determined by master cylinder bore, brake pedal ratio, shoe clearance, and pad clearance, that's why I ask.
Panic stop, we need to know what it actually does?
Rotor surface is controlled by pad material, harder pads polish, softer pads wear out and make dust.
Vacuum going to booster can effect how well the booster performs, low vacuum = low assist.
Master cylinder plumbing, causes wield braking issues, they stop, but not very well. I have done this.

Any pickup should be able to lock the rear brakes fairly easy, even stock drum brakes will. Yours should no matter what is on the front axle. Master cylinder pressure should be checked before changing bores to see what you have now. I bought a 5000psi gauge for this, but didn't need it once I saw the error of my plumbing mistake at the master cylinder.

2 psi residual valves take up pedal travel, they do not improve braking performance. If you quickly pump the pedal, does it get stiffer and stop better? Does it always build pressure at the same pedal travel distance?
If the travel distance changes with a quick pump or two, then residual valves will help, but they won't change how the truck stop. They will only improve performance if the master cylinder is towards the end of it stroke before full pressure is met, but the quick pedal pump will show this. You would also have a very long brake pedal travel if this was the case.

I run a low mounted master cylinder with manual disk brakes with out residual valves and can stop just fine, every time at any speeds. I had to install an adjustable proportioning valve to limit rear pressure just so the rears didn't lock up under hard stopping.

That's a lot of questions. I'll try answer the best I can.
1. Do not have to pump pedal at all
2. I have tried panic braking at 35, 45, and 55,
3. Pedal travel is 3/4 to 1" of free play the moves another 1 to 1/2" under hard braking
4. The pads are semi metallic with clean smooth rotors replaced 5000 miles ago
5. The pads have 5000 miles on them 3/4 left
6. Never run out of vacuum because I put on a reservoir next to the booster
7. The plumbing on the m/c and proportioning valve came pre connected from CCP with pre bent lines
8. When you stand on the pedal hard it is 3 1/2 " from the floor
9. Pedal travel changes 1/2" when you pump the pedal, I attribute that to adjustment of the rears that I haven't done in about a year


I hope this gives you some help Joe
Thanks Sharon

Joe is having the same trouble I am having. He has asked the main question that we don't not understand. He asked:

Panic stop, we need to know what it actually does?

This was what I was asking way back when I said I didn't understand you said it stops at 45 but you are afraid you may rear end someone. Joe was looking for spongy or pedal travel complaint. You have said the "pedal is great" and "no rubber on the road". Everyone is advising to change this and that. It seems everything is new'ish. I looked up a 82 K10 and told you that system uses 1 1/8 bore. you say receipt says 1 1/8 bore.

The convention for residual valves for a low mount MC is 2 lb in front and 10lb in rear. I have wondered if maybe there is nothing wrong without saying so. I have tried to read and reread to see what the complaint is. This is what we always do. We try to understand the poster's problem. Now I an going to say as gently as possible: We don't know what you see as a problem. We don't know if the truck stops normal or not. If the truck does not stop in a normal distance some folks think it's because there is not enough pressure. I recommend adjusting your rear brakes real tight, add residual valves and bleed the brakes. We must get back to a "boiler plate" condition before moving forward.

Regardless of anyone's opinion, the brake system sellers always say to use residual valves in low mount MC/proportional Valve.

Also please describe exactly what concerns you other than: "I'm not saying inadequate but just not up to what I expected."
I hope you can understand we can not react to that kind of comment and are not being rude to you.

We should not start redesigning your whole system.

We are trying to help and we will not give up, but it's hard from our house. So please take these actions in red and report.

Just your information and what we are trying to get, my truck, '37 1/2 ton, 250 inline six ( light weight then OEM engine ) automatic trans, and '79 Trans Am rear axle and drum brakes. Front axle is stock with a Master Power Brake disk brake kit that uses '70-'79 Camaro/Firebird rotors and calipers. I have Ford 7/8" master cylinder.

When I first got it road worthy, I went to a local parking lot and made a lot of panic stops. I wanted to know what the truck was going to do,and did just what I thought it would, pulled a little right and locked up the rear tires really easy. It left black marks from the time I hit the peddle till it came to stop. I then added a proportioning valve I could adjust to limit the rear braking force. I went back to the parking lot and started making 40 mph panic stops, adjusting the valve till the rear brakes just started to lock. At that speed,the truck stopped straight and as quick as it could without leaving black marks. I adjusted my driving style to match the braking capabilities of the truck.

This why we ask, how does it stop.

Your pedal sounds fine, not to much travel and not bottoming out. Not knowing you, it could just be that you are not strong enough to lock the brakes, my manual brakes takes some effort.
Your pads and rotors appear to be wearing OK, you could try another set of pads as a test. Go to NAPA and get the cheapest one they have, they will be soft and have low mileage warranty.

Good luck,
Joe

I'm going in the morning to pick up a set of organic pads and swap them out. Go down my favorite lightly traveled country road and do a bunch of stops and get back to you all. Appreciate all your feed back and working with me on a non traditional set up. You don't know what's going to happen when you veer from the factory set up. Those engineers have it figured out before you take delivery of your truck.
Thanks again
Sharon

For those that occasionally carry loads, 1 ton GM, and probably others, have a proportioning valve near the rear end with a lever hooked to the axle, so when loaded, it gives the rears more braking power.

Ed