The car is a 71 Charger that originally came with non-assist drum brakes with dual reservoir master. I bought the Front disk brake conversion kit from Summit made by SSBC.
http://www.summitracing.com/parts/ssb-a156-1/overview/year/1971/make/dodge/model/charger

It came with the booster, master cylinder, adjustable rod, a front/back bias valve, and the brackets and calipers.

My problem is that the pedal is soft almost all the way down, then very hard. I have to push very hard to stop. Its like there is nothing, then everything. I bench bled the master twice. I bled the system 3 times. I checked the rod length. I put a vac gauge on the booster line and have 17in of vacuum. Here's the thing, This only happens when the car is running. When its not running the pedal is like it should be when it is running and the brakes engage with very little pressure. The supplied bias valve is installed after the brass block in the rear line, like its supposed to be. Its using the original brake brass distribution block but according to the kit, you do use it.

What do you think I should try next?

Thanks

On a toughie I would cap the MC ports with a pair of brass inverted flare plugs from the Edelman fittings cabinet at your parts house. then with eng idling if the MC is holding psi (it s working good) & the MC is bled out adequately and the booster/check valve is functioning normal the pedal will be rock hard with virtually no travel. this'll tell us where to start. Discs can be hard to bleed out on occaision

Do you have the factory belcrank that goes between the brake pedal and the booster? Trouble with those kits is they are a bag of mixed parts from different kinds of cars. You could have the real deal factory parts for less money.

Agreed, the SSBC front disc set is a collection of mismatched parts not properly sized/tested, I'd toss the booster and master set up for factory Mopar related pieces, the calipers/rotors are usable

I'd put a vac gauge on the engine manifold, chances are your running an automatic transmission?, correct? if so, and your manifold vac is around 10-12 inches, the first "braking" IN GEAR, will pull down the vac reading, and repeated braking after that renders the booster with no vacuum assist and no brakes, if this is the case then you might need a vacuum reserve canister, and or vacuum pump

Is it safe to assume that you've properly adjusted the booster pushrod airgap/clearance to master piston bore correctly?

Rapid Robert: Thanks I'll try that this weekend

71 Bird: There is no belcrank There is just a rod that attaches to the pedal and the rod coming out of the booster through the firewall.

Dayclona: Yes auto trans. I put a vac gauge on the booster hose from the intake manifold and have 17in but that's in park. I didn't test in gear. I'll do that.
I adjusted the rod by leaving the master nuts loose, so that when I pushed the pedal the master was pushed forward after moving the pedal just less than the amount it needs to move to release the brake light switch. Then tightened the master nuts. That was without the car running. Maybe that's too far away and the rod is not pushing the master plunger far enough. You adjust the rod between pedal and booster. I don't think there is an adjustment between booster and master. Maybe there is, I'll have to check that also.
As a question, the rod length is the rod length. I thought that the booster only assisted to make it easier to push the pedal, but doesn't have any effect on the fact that the rod is engaging the master cylinder internal plunger, and forcing the fluid. A lack of vacuum would just make the pedal harder to push. Again that's a question. I'm I wrong? Or is it that the rod going in, and the rod coming out, of the booster is not one connected piece?

Thanks all.

the pedal rod and the booster rod are not one, a diaphram(s) a combo of springs, and plates depending on the booster style "link" the two rods together, but you need to confirm that the booster rod does not have excessive travel inside the master cylinder bore that will result in no/low brake pedal, or repeated pumping to stop the car, no clearance will result in the front and or front and rear brakes dragging/locked without your foot on the pedal

What Dayclona said: split the MC from the booster & check the round nub clearance. its an easy fairly non invasive check & might be the fix.

If I could up load a picture I'd show the belcrank that non Hemi 71's with power brakes have. Not been able to upload since the new format on here though. Sorry. Maybe someone else can put up a picture.

...70 B body, but pretty much the same deal in 71

Thanks, I was hoping you had a picture of it.

My car never had an original booster. It was non-power drum brakes. The kit rod goes straight from the pedal to the rod coming out the back of the new booster.

I did the vacuum check with the car running in park and in gear.
17-14" in park. Drops to 7-5" in gear. Guess its safe to say we found the problem. Now, I guess I need a Electric Vacuum Pump Kit. Think I'm going to get this one.

http://www.summitracing.com/parts/sum-760152

I guess I've been under the faulty assumption that low vacuum would make the brakes hard not soft.

Thanks all for the help and more importantly the new knowledge about this stuff.

Oh, here's the car nearing the end of a 100% complete rotisserie restoration.

On further reflection this might be a better way to go. Just ditch the power brakes (master and booster) for manual. If I do that will I just have to replace the master cylinder? And if so, which one would you use? While retaining the SSBC kit calipers and rotors. I'd rather not have to put a vacuum pump on there.

I just went through the same thing on my '71 Challenger. The car was a manual drum 318 car, and I converted it to a 340 front disc car. The 340 was mostly stock, with a slightly hotter cam that would still allow daily driving. I swapped in ALL factory parts for the disc conversion. Still had super soft pedal. Vacuum maxed out at 16 at idle, but I never checked in park. I was about to trash the factory power setup, but with a lot of help from this forum I was able to diagnose the problem. It turns out that the pushrod from the booster had too much travel before making contact with the M/C. I never even thought to look at that as an issue because I bought the booster loaded and figured that would have been adjusted properly. After some adjustment, now I have power brakes! Check the pushrod distance and make sure there is not too much play.

5-7" of vacuum in gear isn't going to cut it with an automatic, you need a minimum of 12 inches/vac for repeated braking, and the booster/engine to recover, a manual transmission can get by with 9" vac, as the engine recovers more quickly because you clutch when braking and the rpm's increase because of no load on the motor when using a power brake system, a pump and reserve canister could be incorporated to boost the brake booster, you could hide it inside the front fender behind the headlight bucket, and the reserve canister under the battery tray so that just some vacuum lines are present in the engine compartment


If you want to go with a manual master, I'd suggest a 1972 Mopar (Barracuda) application M1523 has a 1 1/32 bore is intended for MANUAL DISC BRAKES, you can find M1523 from a Duralast dealer like Autozone/Advance Auto and you can probably use a lot of your original pedal linkage/firewall plate parts to install it


hope this helps
Mike

I use the SSBC front disc conversion on my 68 Satellite . originally a power brake car , due to low vacuum I removed the booster and went manual brakes . I did copy and fab up a manual firewall plate . I do have the master cylinder ,disc/drum , ssbc provided when I bought the kit . I think it may be 15/16" bore , has a good pedal height and feel .
the kit is 4 piston aluminium calipers which appear to have been discontinued . it is a copy of the 66/67 ford caliper I believe , takes that pad
due to drag racing I will likely go to a wilwood caliper/kit in the future

Tex