Brake Issues-Update. We have brakes

A few years ago I got my 68 Charger resto mod on the road. It had manual drums. I converted it to power/front discs. Because of the cam I have low vacuum so the brakes are marginal. After several suggestions from other Mopar friends I installed a vacuum pump. I now have plenty of vacuum at the power booster but the braking hasn't changed. now when I step on the brakes I get a loud Groan from the brake system. (I have a video, too bad I can't play it) It sounds like it's coming from the proportioning valve. I have bled the brakes several times so I can't imagine any air in the system. Any ideas?

When you converted to the disc/drum brake system, did you also change to the disc/drum proportioning valve, or do you still have the drum/drum proportioning valve on your car?
The disc brakes need more fluid then drum brakes do, if you still have the drum brake proportioning valve, your not getting enough fluid to the disc brakes. Gene

excessive pedal travel/hard pedal/not enough braking/do they pump up. More info please.

My first question is about the components used.
What is the master cylinder diameter?
What are the caliper piston diameters?

There are some kits available that have mismatched master cylinders. I know SSBC was one of the big culprits there, as well as some others who used the Corvette style masters.

I've never heard a groan from the hydraulic system on brakes. Boosters can leak vacuum and make noise and of course the friction materials do it, too. Is it possible you're hearing a problem with the bottom of the brake booster diaphragm?

I bought the kit from The Right Stuff Detailing. It's the same kit that several other companies sell. Picked it up at Carlisle in 2013.
Here is a link to the kit. https://www.getdiscbrakes.com/mdc62dc
When I bought it they said I didn't need to switch proportioning valves. After having brake issues I bought a proportioning valve from them. The one thing I thought was wierd was that the passages in the new valve were smaller than the ones in the existing. The new valve made no difference. I'm now wondering if they sold me the wrong one.
Another issue is the size of the booster. It takes up a lot of real estate under the hood making it almost impossible to change the spark plugs.
I am contemplating buying a Bendix set up from Dr. Diff. It would clean up the look under the hood and make the driver side of the engine much more accessible.
Feets- It's possible the sound could be coming from the bottom of the booster, and now that I'm thinking about it, after I changed proportioning valves I remember hearing that groan a couple of times, but much less noticable. It's hard to hear with the motor running. With the vacuum booster I just need to turn the key to on without starting the engine to have the vacuum pump kick on and give me vacuum at the booster.
I don't have a problem with a hard pedal or pedal fade.

When you say the brakes are marginal do you mean that it takes both feet on the pedal and hour butt in the air to stop the car?
That kit lists a 1-1/6" master cylinder. That's a big master for a stock style caliper with a manual pedal. Did your pedal have two holes for the master cylinder push rod? If so, check the position and make sure you've got it set correctly.

booster pushrod to MC cup clearance OK? (~.020")

THe first thing I checked. I even adjusted it both in and out to see if it made any difference.

Feet- No matter how hard I stand on the pedal I can't get the brakes to lock up. Just one hole for MC push rod

either there is a fluid restriction somewhere or a mismatch on the bore sizes which I ain't telling you anything you dont already know, I'm just thinking out loud...

Have you verified the booster is working???

Next verify the vacuum that the pump is producing?

is it the only source of vacuum and if not is it plumbed properly?

Do not overlook the pad compound. We have had several kits from different sources that have displayed poor performance. After verifying pressures at the master and calipers swapping in pads with more aggressive compounds did the trick. OF course the suppliers always claimed we were the only ones that had experienced a problem. Sure!!!!

That's what we needed to know.

Power conversions are tough to get right with a manual pedal.

Your master cylinder is too large for your pedal ratio.

When you push the pedal there is not enough leverage to get all the fluid from the fat master to work the calipers.



Read this thread: https://board.moparts.org/ubbthread...ou-never-wanted-to-know.html#Post1235219

Measure your brake pedal ratio. If it is a typical 7:1 ratio then you're going to need to change it or get a smaller master cylinder.

TPJ- I checked the vacuum at the booster, over 20. I tied the electric booster pump into the line that comes off the manifold in case the pump ever fails I don't want to be left with no brake vacuum. I also put a check valve in the manifold line so as not to upset the engine vacuum. Never thought about check brake pressure at booster and calipers, I will look into that.

Feet- That's quite a thread. I will definitely check the pedal ratio.

Thanks for all the feedback.

pump the pedal several times to use up any residual vacuum.

Apply a steady pressure to the pedal and turn the pump on. If the booster is working the pedal should drop with the vacuum assist. IF not that is likely your issue.
If it does drop, next step is to put a gauge on the master, and then the calipers to see what kind of pressures you're generating.

Feets- measured the pedal ratio. 7.037 I also checked my 72 Charger that came with front discs. 7.22 pedal ratio. I'm assuming you are saying I need a longer brake pedal for additional leverage. The pedal ratios tell me the pedal in my 72 is 3/16" longer than my 68. How much longer of a pedal would be necessary? Another thing. The brake pedal in my 68 sits very high, my Mopar buddy said that is typical of a manual drum set up???.
If the pedal was lower, closer to the floor would that help since the angle of the brake pedal would be closer to 90º to the master cylinder push rod?

TJP- I will check that this weekend when the temps are above freezing for a change. What type of gauge do you use for testing the master and calipers?

0-2000 psi

All good information here. Also, I know this sounds dumb and you may have done this already, but remove the pads and look them over. I had a small leak on the inside piston (4 piston calipers on my 69) and it would leak enough to soak the pad over the winter but not enough to leave a puddle or tell tail. I cleaned up my rotors, replaced seal and pads, and much better. I had my power booster take a dump many years ago so went with manual discs. Not pleasant and kept me from enjoying the car. I just sent booster out for rebuild. Hoping to put many more miles on it this season.

Have you verified the booster yet ?? If not I would do that before spending the $$ on the below kit.


A kit a similar to this

Hydraulic/ brake pressure test kit

You should be Generating 1800+ PSI on a typical brake system during a panic stop at the master and calipers. I typically like to see 2000+.
There are a ton of variables when it comes to those numbers but these are typical of my findings with disc brake system from the 60's- 70's and the conversion kits.

Keep us posted

It sounds like your parts are working as designed.

They're just not working well together. The components are not well matched and I've seen this before in these kits. The master is too big for the pedal and calipers.

They do not list the caliper piston diameter. We need that info to find out what is going on. I bet a smaller master will fix the issue.

Since it was still a balmy 38º I ventured down to the garage to try what TJP had suggested.
I pumped the brakes for a bit, then held my foot on the pedal with steady pressure and turned the electric vacuum pump on. The pedal did not move.

Assuming the above test was done with steady pressure on the pedal, And
that the (the vacuum pump) ran until it shutoff (15" +), And
the pedal did not drop with the same steady pressure applied, That would indicate a bad booster. It may also explain the "groan" noise you described.

The booster is supposed to multiply the pedal pressure applied by using the vacuum. The above test should cause the pedal to drop as the vacuum is applied thereby generating more hydraulic pressure with same pedal effort, hence the term "power brakes".

hope this is making sense

sounds like the booster is bad

TJP- I doubled checked yesterday. I tested the vacuum at the booster (26"). Yesterday was the first day since I installed the vacuum pump that I could test the car in the driveway, not just the shop. The vacuum booster made a marginal difference. so I definitely still have a problem.

I went onto the getdiscbrakes web site to try and find some more info on the brake calipers. When you click on the link to the brakes a message pops up saying they currently are not stocking brake calipers for 68 Chargers at this time. Wonder if they knew they had a problem.
As far as buying a new booster I'll look into this one from Dr Diff. https://www.doctordiff.com/bendix-style-dual-diaphragm-booster.html
I'm also thinking I should buy a new proportioning valve. As I mentioned earlier the passages in the new one that Fine lines sent me were smaller than the one that was matched up with my manual drums. I'm now questioning everything that I got from this company.

correct me if i'm wrong, as i'm looking at my 69 FSM. the drum booster is a single diaphragm, while the disc booster is a duel [tandem diaphragm the FSM calls it] unit.
if you are using a single diaphragm booster, it probably doesn't have enough beans to work with the disc setup.
the good doctors dual diaphragm looks like what you need.

Single or dual diaphragm can get confusing if one does not know the size of the diaphragms. The size ( coupled with the vacuum) is what multiplies the force applied at the pedal. Dual diaphragms are typically used due to space constraints or when more pressure multiplication is needed..

I was just thinking, you can use one of your daily drivers to repeat the booster test assuming they have power brakes. As before, pump the pedal several times to empty any residual vacuum. Apply a steady firm pressure, start the car and the pedal should drop. it will give you a "feel" for what you should expect with the other system.

As far as the porp valve goes they are sometimes nothing more than a pressure differential valve that are mislabeled.

A proportioning valve will control front to rear pressures

A differential valve does not, is is only there to activate the dash light if the valve sees a drop in pressure on one side. it's other purpose is to split the front and sometimes rear lines.

Different hydraulic systems control pressures a multitude of ways, pedal ratios, boosters, MC bore diameters ( Some are stepped internally to provide different pressures to the front and rear), Piston diameters on calipers and wheel cylinders, (Again some wheel cylinders are stepped for the front and rear shoes on the same wheel as are some multi piston calipers, The surface area of the drum/rotor, the material they are made from, and finally Pad/shoe size and friction compound(s) used

Confused yet ??? Hopefully not, keep us posted

I bought a new Bendix booster, M/C and proportioning valve from Dr. Diff. Finally got to test drive today. Wow- I actually have working brakes.
When I removed the old proportioning valve and compared it to the new one the passages in the new one were twice the size. After installing everything and turning on the vacuum pump and pressing the brake- no more groan from the booster. Went for a 20 mile test drive and then checked the temps on all the brakes with my infrared temp gun. all the brakes were cool, nothing hanging up or dragging.

Thanks to all for the advice/input.

Glad you got it handled ad thanks for the update

what is the diameter of the bendix booster ? [TIA]
glad you have solved the problem !