Hi! I am working on a 70 Challenger. Has poor brake performance with all type parts tried. Different boosters, MC's, calipers etc.
Currently has 2-3/4" (or so) chrysler slider calipers, 11,75" rotors front, Bendix style booster and a proper MC for the set-up (all from Dr.Diff).
Rear discs from Ssbc. Wilwood adjustable prop. valve in rear line and a disc-disc combination valve.

Brake feels normal and grabs great for light breaking, but when pushing the pedal further it is like hiting a wall. Rock hard pedal and not nearly able to lock the brakes on loose gravel! Flat out dangrous to drive in traffic.

I have tried all different settings on the prop. valve without change.
Brake warning light works , but does not illuminate, so I don't think the valve gets tripped.

Feels like the rear brakes are hydro locking and prevent pressure to the front.

Bleeding fron brakes is easy and normal. Pedal will bottom out when pressure bled.
When bleeding the rears I noticed that the pedal not will bottom out but stop where it stops when all is bled, about half way through total travel.

What am I missing here?
Only things not changed is combination valve, prop. valve, hard lines front and calipers, hard lines and hoses in the rear.
All boosters tried worked great and I even plumbed in a vacuumn pump that pulled 20hg to make sure there was no problem in that eria.

Had the combination valve open but could not spot anything obvious wrong.

Really ned help with this one!!!

Thanks

What SPECIFIC rear disc brake kit from SSBC do you have?

What SPECIFIC pads are you using, front AND rear?

disc/disc combos do not need a combination valve. Those are designed to mate the different operating characteristics between disc fronts and rear drums so that you have proper operation.

Can't answer the question, or help, if the info given is generic.

This part is contradictory
Feels like the rear braked is hydro locking and prevents pressure to the front.

Bleeding fron brakes is eaay and normal. Pedal will bottom out when pressure bled.
When bleeding the rears I noticed that the pedal not will bottom out but stop where it stops when all is bled, about half way through total travel.


If you bleed off the fronts and it goes to the floor, but bleeding the backs it stops, then it must be the fronts causing this not the rears.

Eitherway, I might put it up on stands on all 4 coroners, and have a friend hold down the pedal and see if any of the brakes act differently. being able to move around without changing anything should let you see if something is off.

Adjust the tip of the master cylinder push rod so it is almost touching the base of the piston at rest.

Get rid of the adjustable prop valve.

Make sure you have a simple distribution block (no tall brass nut at the top).

Make sure the cables are not preloading the parking brake mechanism in the rear calipers.

Ratchet the parking brake mechanism before bleeding the rear calipers.

Thanks Doctor😀
-Maste cylinder push rod is adjusted like you describe. There is no signs of the MS bottoming out.

-The adjustable prop. valve has been there since I got the car. Can they be troublesome? My believe was that it should not come into play when fully open. I might very well be wrong😋

-The combination valve has also been in there from the start. It is the disc-disc type with no metering valve for the front, but a proportioning valve to the rear and ofcourse the pressure differential valve with the connector for ground signal to the brake warning light.
My understanding is that the prop. valve only delays fluid to the rear when panic braking. Am I wrong?

-I will check parking brake cable tension, but I thing I got that covered already.

-Can you explain "ratchet the parking mecanism"? I am Norwegian an might be a little slow😂🤣
I heard something about the rear calipers that are adjustable could cause trouble and adjusted them pr Ssbc's instructions for a gap between pads and rotor to 1/6 to 1/32".

All parts I got from you are great and out of the question😀 But they did not solve the problem. They were only purchased to optimice the system.

Front lines, rear lines and hoses, rear calipers, comb. valve and adjustable prop. valve is what is left in the system from when I got the car.
To me it boils down to problems with valve(s) or rear calipers.
Why will the pedal go to the floor when bleeding fronts but not the rears?

Hope yo hear from you 😀

so a few things about what doc is saying.

First, the master and booster.
If the booster push rod is adjustible, it may be too long and engaging the master cylinder BEFORE you push on the pedal.
to check this, you have to unbolt the master from the booster.
measure the length of the rod coming out of the booster and then measure the depth of the master cylinder.

How I do it is I pull about 3/4 of the fluid out of the master and make sure what is left in it is new so I can see through it.
Powerful flash light. there are 2 holes in the bottom of the master you can see into and watch the piston in the master.
When you unbolt the master and pull away, you can watch for the piston to move. When you push the master back against the booster, you can watch it for movement then as well.
If it moves the rod is too long and you have to move the adjustment to make it shorter.

Second ratcheting.
It is just to engage the parking brake 1 or 2 clicks when you push on the parking brake pedal. enough that it engages the pads to the rotor. before you bleed.

Aha! Thanks. I have not ratched the parking brake before bleeding the rear. I need to investigate more around the rear calipers.

For the MC: I have checked for freeplay but I will check again. I doubt this is the cause because I have tried three different MC's and always checked and adjusted for correct freeplay. What exactly happends if the rod is a tiny bit too long? Will it only cause pressure to stay in the system and finally create a drag? Or can other things happen?

You might want to keep the adjustable proportioning valve in the rear line if you can't maintain a (front/rear) 60/40 - 70/30 brake bias based on the existing caliper sizes you have, ditch the disc-disc combination valve, as far as the SSBC rear calipers, make sure they are centered on the rotors when installed, most people just bolt the calipers to the brackets, unaware that the caliper must be installed centered on the rotor, the SSBC kits use washers to shim the caliper where it's required, unfortunately SSBC kit install instructions neglect to mention this....

MikeG

If the rod was too long wouldn't that just block the compensation ports when brakes were off keeping residual pressure on and start to drag brakes because piston did not return fully?

Can you post a picture of your combination valve? If a late model that combination valve has metering and proportion all ready. You state you have a proportion valve in rear with brake light warning? What about the combination valve warning?

I think you just need a distribution block as mentioned. That just distributes flow and it will have warning light for failure of one side. Then with your adjustable proportion valve you can set your balance.

A Mopar MC has the primary piston for the front brakes at the rear of MC (large reservoir). The smaller secondary piston is at the front of the MC. If you open the front brake bleed valves and press brake petal the front piston can collapse to the rear piston and full travel. When you go to bleed rear, and the pedal stops that is because the front piston seems to lockup before it collapses enough to move the rear piston sufficiently.

So either you have issues with the combination valve or some other problem preventing sufficient pressure build up at either set to even lockup a wheel in gravel. Pressure gauge can help you determine where the issue is. But your statement car never had good brakes and you still have your combination valve original, would lead me to get rid of it, and get the plain distribution block in.

You can gravity bleed and I use a small inexpensive vacuum pump kit to bleed brakes.

Thanks!😀
Warning switch is in the combination valve. This valve only has a proportioning valve at the out port for rear brake line. No metering valve for the fronts at it s a disc-disc design. So other then housing a warning switch and a prop valve to the rear to awoid rear locking when panic braking it only functions as a distributor block.

I does not build pressure or create a drag.
I agree about removing the combination valve.

I jus wonder if the rear calipers can play a part here. They are adjstable and rests with a gap between rotor an pads. I never ratceted the parking brake before bleeding. Wonder if there is air trapped in the calipers making the pressure differential valve trip during hard braking.

As stated brakes feel super nice under light braking. No extra effort needed to slow down calm, but there is just not more stopping power to get.

There should be no gap between the pads and the rotors.

Pr Ssbc's instructions it should be set between 1/16 and 1/32".

SSBC instructions are incomplete or wrong.

That section is the parking brake adjustment. That is where you START to properly adjust the parking brake, not where you end up.

You need to cycle the parking brake several times till that clearance goes away and the parking brake actually functions. then you should have brakes.

Sniper:
You are right about that. Pads are tight to the rotor now. Just checked. And parking brake works.
I racheted the parking brakes and vacuumn bled the four corners from rear right to front left and it took the fluid a couple of seconds
to come out of the rear calipers and I saw some tiny air bobbles at the start. The calipers have two bleeding screws each. Don' t know why. I bled both to be sure.

Took the car for a test drive and noticed an improvement. It doesn't feel scary to drive and stops fast from 50 mph. But the last "oomp' is missing. It barely locked the rears on hard gravel when I steped on the pedal. This was with the adjustable prop. valve fully open.

I also noticed the pedal would depress a little more now. Like it is pushing more fluid to the front. Car wil nose dive good without pushing har on the pedal.

Still puzzeled why the last braking power is not there.🤔

I wonder if the prop valve for the rears in the combination valve can play a role. Wish I had a plain distributor block to swap in to test. This valve is the only thing left I can think of acting up.

I start bleeding the farthest from the master first (back) bleed one back then one front, both while pushing the pedal. That let’s ALL the fluid move out of the master cylinder. Sounds like you still have air in the back, reason it goes to the floor when bleeding the front but not when you bleed the back. Remember,Two separate chambers in the master, one rod pushing both.

Where did you get the disc disc combination valve? Not what would have been on a 70 E body. If you have 2 proportioning valves in the system, yes you will wind up with less pressure at the rear. Even so the front should be working fine unless you have air in the system still.

Yup. It is the front that seems to not get the pressure it should, but the rear might take mor too, if not the prop valv in the combination valve is just doing it's job and preventing the rears to lock up.
The combination valve came with the car whe I got it. I believe it was part of a complete Ssbc 4 corner sisc brake kit with a 2-bolt GM style Master cylinder and booster. Only thing left now is valvea and rear brakes.

Attached is a pic of the combination valve that is in the car.
It works as a T for the front brakes. Other than that it had the warning switch/valve and a proportioning valve for the rear brakes.
In wich way could this piece act up and cause a hard pedal?

As I don't have a distributioning block handy, is it possible to remove the rubber plunger inside behind the tall brass nut to eliminate the proportioning feature to test if this is the problem? Eliminating the proportioning should make it function as a simple distributioning block with a warning switch/valve.

I still don' t see how a valve like this could reduse pressure to the front.

Pressure gauge and test at various points can help you divide and conquer. We are arm chairing this at this point.

Agreed, when I set up a 4 wheel disc brake system of my own design or pre-assembled kits for customers or myself, regardless of the manufacturer's claims I employ a high pressure gauge at each caliper to test how biased/balanced the system is rather than seat of the pant's guessing/trail and error, Summit Racing offers an affordable testing gauge kit for those on a budget as 2000psi gauges aren't exactly cheap if you want to make your own 4 wheel testing system


Mike