4 wheel disc troubles

-
kyles73dart

kyles73dart

Well-Known Member
Joined
Mar 13, 2011
Messages
156
Reaction score
21
Location
Dawsonville, GA
I've read quite a few threads about this and the amount of different information I've read has me not sure where to begin. First, here are the brake system specs:

- Power Brakes
- 1.032" Bore MC
- Front discs are from an Aspen I believe (If not, they're from an A-body)
- Summit disc brake conversion kit Summit Racing SUM-BK1330-99904 Summit Racing® Rear Drum-to-Disc Brake Conversion Kits | Summit Racing
- Brand new SS brake lines and new brake hoses
- Drum brake distribution block

My issue is the braking is very poor front and back. In order to stop, I have to push the pedal as far as I can (pedal doesn't touch the floor) and it'll slowly come to a stop. I tried to do some panic stops on our private road and I can barely lock up the fronts if I stand on the pedal with both feet going ~20mph. It will lock the fronts up on gravel at a slow speed but I still have to firmly push the pedal. If I pump the pedal it seems to firm up some as well.

I've bench bled the master a couple times and I'm positive there is no air in the system. I've adjusted the pushrod between the booster and MC and that didn't help any. I'm leaning towards an improper MC for the setup but I don't want to go spending money where it isn't needed. I appreciate any help that can be provided and let me know if I missed anything important.
 
kyles73dart said:
I've read quite a few threads about this and the amount of different information I've read has me not sure where to begin. First, here are the brake system specs:

- Power Brakes
- 1.032" Bore MC
- Front discs are from an Aspen I believe (If not, they're from an A-body)
- Summit disc brake conversion kit Summit Racing SUM-BK1330-99904 Summit Racing® Rear Drum-to-Disc Brake Conversion Kits | Summit Racing
- Brand new SS brake lines and new brake hoses
- Drum brake distribution block

My issue is the braking is very poor front and back. In order to stop, I have to push the pedal as far as I can (pedal doesn't touch the floor) and it'll slowly come to a stop. I tried to do some panic stops on our private road and I can barely lock up the fronts if I stand on the pedal with both feet going ~20mph. It will lock the fronts up on gravel at a slow speed but I still have to firmly push the pedal. If I pump the pedal it seems to firm up some as well.

I've bench bled the master a couple times and I'm positive there is no air in the system. I've adjusted the pushrod between the booster and MC and that didn't help any. I'm leaning towards an improper MC for the setup but I don't want to go spending money where it isn't needed. I appreciate any help that can be provided and let me know if I missed anything important.
Click to expand...
Do you have a proportioning valve in the system?
 
How much vacuum does your engine make at idle? Stock or aftermarket booster? Booster Size?

That rear kit is basically a 87-88 Thunderbird Turbocoupe setup. Do you have parking brakes set up?

upload_2021-1-25_6-51-14.png
 
Last edited:
Bleed the **** out of it, check for fitting leaks. What diameter is the front pistons? That needs to be matched to master bore diameter. No prop valve, try a wilwood adjustable proportioning valve.

You may need a hold off valve in the rear circuit. This is in effect a 1 way check valve. It ports fluid 1 way and keeps the rear pads very close to the rotors when you release the brake pedal. This helps prevent having to pump multiple times to get them to come out.

If using A body or F body calipers they are 2.60" diameter pistons. You may need to either increase the size of the calipers to 78-79 B-C-M calipers with 2.75" pistons, or if you dont want to do that, try decreasing the master cylinder bore diameter to a smaller bore. Something under 1".

Somebody may know more than me on this subject, but typically a larger bore master makes a hard pedal and calipers that dont grip tightly, and a smaller bore master cylinder with the same calipers will give you a softer pedal but better grip.
 
Last edited:
dano said:
How much vacuum does your engine make at idle? Stock or aftermarket booster? Booster Size?

That rear kit is basically a 87-88 Thunderbird Turbocoupe setup. Do you have parking brakes set up? Those calipers use the parking brake for adjustment. If not adjusted properly you'll get poor braking.

View attachment 1715677039
Click to expand...

Booster is a stock reman unit like this: https://www.autozone.com/brakes-and...-reman-brake-power-booster-54-76101/44696_0_0
As far as engine vacuum goes, it's been a long time since I measured but I don't recall it being low. I can measure it when I get home later.
Parking brake is adjusted, had to loosen the adjuster almost all the way to keep it from dragging. But I can check it again.

moparmat2000 said:
Bleed the **** out of it, check for fitting leaks. What diameter is the front pistons? That needs to be matched to master bore diameter. No prop valve, try a wilwood adjustable proportioning valve.

You may need a hold off valve in the rear circuit. This is in effect a 1 way check valve. It ports fluid 1 way and keeps the rear pads very close to the rotors when you release the brake pedal. This helps prevent having to pump multiple times to get them to come out.

If using A body or F body calipers they are 2.60" diameter pistons. You may need to either increase the size of the calipers to 78-79 B-C-M calipers with 2.75" pistons, or if you dont want to do that, try decreasing the master cylinder bore diameter to a smaller bore. Something under 1".

Somebody may know more than me on this subject, but typically a larger bore master makes a hard pedal and calipers that dont grip tightly, and a smaller bore master cylinder with the same calipers will give you a softer pedal but better grip.
Click to expand...

They are definitely A or F body calipers
 
If you are using an Aspen disc setup, use an Aspen Master cylinder and/or get the specs on that m/c and you may need a 4 wheel disc master. Disc brakes require more brake fluid in reserve that drum brakes because of the smaller volume of wheel cylinders vs disc brake pistons.
Did you get the info for the swap from a magazine article ?
For others FYI. If doing a hydraulic swap it is best to get the whole setup, Calipers, M/C, Any Prop valves etc. Since we know they were proper from the factory it is sometimes hard to "Frankenstein" a system of different parts not knowing the compatibility of the parts yo are using now. These are "systems".

Read through this article completely. A lot of little details.
DISC-O-TECH: Stop on a dime
 
Last edited:
MoparLeo said:
If you are using an Aspen disc setup, use an Aspen Master cylinder and/or get the specs on that m/c and you may need a 4 wheel disc master. Disc brakes require more brake fluid in reserve that drum brakes because of the smaller volume of wheel cylinders vs disc brake pistons.
Did you get the info for the swap from a magazine article ?
For others FYI. If doing a hydraulic swap it is best to get the whole setup, Calipers, M/C, Any Prop valves etc. Since we know they were proper from the factory it is sometimes hard to "Frankenstein" a system of different parts not knowing the compatibility of the parts yo are using now. These are "systems".

Read through this article completely. A lot of little details.
DISC-O-TECH: Stop on a dime
Click to expand...

Couldn't find a definite answer on the Aspen MC specs but it seems replacements from summit are all 1.031". This is a snippet from the 1973 service manual on MC specs and if A and F body both use 2.6" caliper pistons then maybe I need the .937" MC?

Capture1.PNG


Now looking deeper in the 73 manual, they flipped the MC bore information from above?

Capture2.PNG
 
Last edited:
I could be wrong, but I think A body power brake master cylinder is 1&1/32" because of the linkage setup required to raise the booster up to clear a V8 valve cover. It may have something to do with that. Manual disc brakes do not have this linkage. This may be part of the reason why those use 15/16 bore master cylinders.

A hold off valve going to the rear brakes may help keep pedal pressure up without multiple pumping. The fluid moves very little if the pads are in very close contact with the rotors.

The Aspen booster setup I believe is bolted straight to the firewall without the additional linkage to raise it up to clear like an A body. I dont see F body specs in your manual pix.

Fyi 15/16" is the same measurement as .937" it's just in decimal and not fractional.
 
Last edited:
Your entire diagnostic can be run in the garage; then a road test will prove you fixed it. I will assume that your front wheelbearings are in good shape, and adjusted correctly. and that your vehicle does not have independent rear suspension. In other words you have a vintage Mopar.
Before you start; read the notes at the bottom.

First, some basic questions;
A) is this an automatic car? What size engine? and what idlespeed? and what stall? What I'm getting at is this; if the engine is pulling hard during braking, and your brakes have to fight your engine, they will lose.
B) with the engine off, pump the brakes about three or more times, to exhaust the booster, keeping the pedal down medium hard on the last pump. Now start the engine. As the vacuum comes up in the booster,the pedal should fall. If it does not, then the booster is not charging. Rev the engine up to 1500rpm, and wait.
Still no falling pedal? then the pushrod length needs to be revisited.
a) If the pushrod is too short, it will not activate the control valve in the booster, and with the big-bore M/C it will take a lotta muscle to stop the car.
b) If the pushrod adjustment is too long, it will activate early and either slam your head into the windshield during soft braking, or it is easily possible to break that control valve, and then you will hear hissing under the dash or at the firewall under the hood.
c) The vacuum source-line for the booster should be plumbed to the plenum, NOT to just one runner, and the booster end should have a working check-valve right at the grommet. And, the PCV should be plumbed to the underside of the primaries. The Idle-Timing has to be lazy enough that the transfers are properly engaged, and the mixture screws close to, in the center of their working range; 2.5 turns on a Carter, and about 3/4 turn on a Holley-type. With a typical street cam this will be no more than 16 degrees

Now the hard questions; on the first pump;
C) Is the pedal HARD and high?
with a small amount of travel?
and does it stay there for at least 20 seconds? If not, this has to be fixed;
1) if the pedal does not remain high, then fluid is leaking somewhere. If not external, then it is leaking back into the reservoir, and the M/C is defective.
2) Does the pedal get harder with multiple pumps, like you are pushing on a big spring? This usually indicates air in the system,but could also be "flex". Flex could be soft hoses or bent pads, or calipers not parallel to the discs, or not enough piston travel.
3) Does the pedal get higher with multiple pumps.
If it does, then you have or may have a mechanical problem like; excessive pad travel, knock-back, frozen sliders, warped pads, or even seizing pistons.
4) If you get to this point, with a proved to be working booster,and a proved to be not internally leaking M/C ....... but with no good results, then;
take all the calipers off their brackets and C-clamp the pistons into the bottom of the bores so that they cannot move; then go back to: C) above, and repeat the diagnostic.

Notes;
>If the rear system has a problem
then the whole system will suffer, This is because of how the M/C works. The front system is hydraulically activated by trapped fluid that in a Mopar system, is gonna activate the rear system first. For it to work properly, the back has to work properly first, or it has to fully fail, and then the mechanical back-up system will make the front work, allbeit with a very low pedal.
>the Disc/disc system if properly sized, does not need any valving, it can run off the drum/drum distribution block.
>For diagnostics, any M/C can be used, as long as there are no Residual valves in it.
>Make absolutely sure that the parking brake is not in the least working
>Before you finish up, make sure the Compensating Ports in the M/C are open, else, over time, as the friction surfaces wear, the pedal will get closer and closer to the floor.
> I have had no success with the dual diaphragm boosters and low engine vacuum. I use one of a late 70s F-body, it works great.

>It is possible that you get everything working exactly perfectly, yet on the roadtest, it fails to stop properly. In this case, I would suspect; pad contamination, glazed pads, overly hard discs, or the calipers are not properly located over the discs. In the last case it might be possible that the calipers are only contacting the discs on one side, versus squeezing from both sides. Yeah that ain't gonna work.

In the most stubborn cases, I have clamped the rear flex line(s) to isolate the front system from the rear, but be advised that on older flexlines, this can damage the line internally, and you might not know it until weeks or months later. This is a quick and dirty test that makes perfect sense when the shop you work in charges over $100 per hour. But for myself, I remove the calipers and C-clamp the pistons to the bottom. If the hoses are good, this works exactly the same, it's just a lot more work.

Ok what did I miss, lemmee think.....
oh yeah; pedal ratio. But with a good working booster, this should not come up for discussion.
 
Last edited:
check to see if the combination valve is working proper if you have one it could have slid over blocking off fluid to the front wheels do you have good fluid flow to the front?
 
kyles73dart said:
They are definitely A or F body calipers
Click to expand...
Don't forget- not all A body calipers are created equal- '73-'75 calipers used a 2.6" piston, '76 A body and F/M/J used a 2.75" piston.
From the factory, '73-'75 As used a .937 master cylinder for power brakes, and a 1.032 for manual.
1.032 MCs are listed for both power and manual brakes for the '76 and up calipers.
Additionally, the early power brakes used a 9.88 dia. booster, '76 and later got a 11.43 diameter diaphragm.
All the little differences can add up. As MoparLeo stated above, stick with the matched factory parts.
Adding rear discs complicates it a bit, but removing your rear residual valve was the right move. At this point, I'm thinking there's still air in the system- it can be real stubborn. You do have your calipers on the correct sides, with the bleeders facing "up", right?
 
I have done a few hundred brake conversions over the years at our shop. I'll will tell you the master is generally the problem. Have had plenty faulty masters come from the parts store and even new ones that had bad o rings ,and the like. Bench bleed the master, and make sure it is even doing its job.
 
AJ/FormS said:
Your entire diagnostic can be run in the garage; then a road test will prove you fixed it. I will assume that your front wheelbearings are in good shape, and adjusted correctly. and that your vehicle does not have independent rear suspension. In other words you have a vintage Mopar.
Before you start; read the notes at the bottom.
Click to expand...

I will run through these tests when I get home and report back.

teringer said:
check to see if the combination valve is working proper if you have one it could have slid over blocking off fluid to the front wheels do you have good fluid flow to the front?
Click to expand...

I do have flow to the front. When manually bleeding the fronts there is enough pressure to hit the inner wheel well and the peddle will drop to the floor.

Professor Fate said:
Don't forget- not all A body calipers are created equal- '73-'75 calipers used a 2.6" piston, '76 A body and F/M/J used a 2.75" piston.
From the factory, '73-'75 As used a .937 master cylinder for power brakes, and a 1.032 for manual.
1.032 MCs are listed for both power and manual brakes for the '76 and up calipers.
Additionally, the early power brakes used a 9.88 dia. booster, '76 and later got a 11.43 diameter diaphragm.
All the little differences can add up. As MoparLeo stated above, stick with the matched factory parts.
Adding rear discs complicates it a bit, but removing your rear residual valve was the right move. At this point, I'm thinking there's still air in the system- it can be real stubborn. You do have your calipers on the correct sides, with the bleeders facing "up", right?
Click to expand...

I can check the calipers when I get home but I'm 70% positive they are F body (2.75"). My booster is the earlier 9.88" single diaphram with the 1.032 M/C. The kit puts the bleeder in a bad position but I did unmount and bleed them with the bleeders verticle.

Resized_20180529_150836.jpg
 
That hose loop is way high, there could be some air trapped in there as well. If you can bleed with the caliper above the top of that hose that may just do it.

Otherwise, disconnect the battery, force the pedal on overnight with a stick or whatever, and then come back in the morning and re-bleed. It could be that some air is also stuck in the ball-ramp mechanism for the parking brake. Hard to tell if you have them hooked up but its good to cycle those also.
 
kyles73dart said:
I've read quite a few threads about this and the amount of different information I've read has me not sure where to begin. First, here are the brake system specs:

- Power Brakes
- 1.032" Bore MC
- Front discs are from an Aspen I believe (If not, they're from an A-body)
- Summit disc brake conversion kit Summit Racing SUM-BK1330-99904 Summit Racing® Rear Drum-to-Disc Brake Conversion Kits | Summit Racing
- Brand new SS brake lines and new brake hoses
- Drum brake distribution block

My issue is the braking is very poor front and back. In order to stop, I have to push the pedal as far as I can (pedal doesn't touch the floor) and it'll slowly come to a stop. I tried to do some panic stops on our private road and I can barely lock up the fronts if I stand on the pedal with both feet going ~20mph. It will lock the fronts up on gravel at a slow speed but I still have to firmly push the pedal. If I pump the pedal it seems to firm up some as well.

I've bench bled the master a couple times and I'm positive there is no air in the system. I've adjusted the pushrod between the booster and MC and that didn't help any. I'm leaning towards an improper MC for the setup but I don't want to go spending money where it isn't needed. I appreciate any help that can be provided and let me know if I missed anything important.
Click to expand...
Sounds to me like you just have more bleeding to do. Have had some that take twenty minutes some take hours to get all the air out. Noticed some high arching rubber lines and such that will make it take longer. I usually crack open all the bleeders and wait for fluid to start flowing than shut them down as it comes out of each caliper. Start at the furthest away from master when bleeding back to front. But you probably know the drill. If doing g it manually make sure the person pumping is paying attention. It takes twice as long if at all when the kid helps. Pump and hold sometimes are tough to do I guess lol
 
You must log in or register to reply here.
-

Back
Top