1969 Roadrunner 4-wheel disc brake conversion - weak brakes

[rdbird9]

So first off I've only had this '69 Roadrunner for a week, so I don't know a whole lot about it. The car was originally drums all around that I believe were power. The previous owner did the 4-wheel disc conversion and said that it works okay but could be better and I agree. It's not horrible in normal driving but I can see how in an emergency situation the brakes could be a bit scary. It seems to take a fair amount of pedal travel to slow the car and even with full pedal it won't lock the wheels up. With pressure on the pedal, it drops when vacuum is applied to the booster so that seems good. I replaced the front hoses (had rubbed on the control arm resulting in a worn spot on each side), adjusted the wheel bearings (they were a little loose), and bled the brakes to no avail. Pedal does not pump up so I doubt there is air in the system.
I'll start off with what I do know about the conversion. It is an SSBC 4-wheel disc brake kit front and rear. The front brakes for this kit appear to be 1967 Mustang style pads, rotors, and calipers with some bearings that allow them to work on the stock drum brake spindle. The rear pads/calipers are for a late 1980's Thunderbird. It has a proportioning valve mounted on the driver's side frame rail under the steering shaft. I have not touched that. The master cylinder looks to be a normal Chrysler 1969 disc/drum style unit. I don't know about the booster but I assume it's probably stock as well. Here's where it gets a bit weird with the booster. It is not connected to engine vacuum but instead is connected to an electric vacuum pump and tank unit mounted in the trunk. There is a metal hardline that runs from the trunk to the engine bay where it connects to the booster with rubber line. According to the vacuum gauge on the dash, the pump kicks in when the vacuum goes below 15in and cuts off again at 25in. The car has a bit of a cam but I doubt it's enough to mess the vacuum signal up, so I'm not totally sure why the pump setup is there, but it does the job.
This is the only disc conversion I've dealt with, so I'm a bit at a loss but don't want to just go throwing parts at it.

Here's a picture of the master/booster assembly which I think is just stock Chrysler stuff. I can take pictures of the other stuff too if that would help with diagnosis.

 

[BeepBeepRR]

Do you know if the previous owner changed that Booster? If not thats not for disc brakes. Its a drum drum booster and is a single diaphram you need a dual diaphram for disc. I may be way off others will chime in.

 

[myk r sanchez]

I thought all the booster does is just provide vacuum assist and reduce effort? I'll be watching this one...

 

[BeepBeepRR]

Copied this from Speedway Motors site.

SINGLE DIAPHRAGM BOOSTERS work well with four wheel drum systems or front disc brakes and rear drum applications. These braking systems are lighter than the four wheel disc systems and work well on older cars because drum brakes were a common feature, at least on the rear axle.

DUAL DIAPHRAGM BOOSTERS are needed for four wheel disc systems because they can produce a higher level of brake assist even if their physical size is smaller. Dual diaphragm brake boosters tend to give extra braking even when the vacuum from the intake manifold is low.

BOOSTER SIZE is an important consideration. Power brake boosters require 17" of vacuum or more to function properly. Smaller brake boosters are great for tight spaces, but may require up to 22" of vacuum in order to function properly.

 

[BeepBeepRR]

This comes from this link https://itstillruns.com/difference-jb6-jb7-master-cylinders-7983594.html

Power brake boosters help the driver push the brake pedal. "The Reader's Digest Complete Car Care Manual" states that most designs employ the use of vacuum from the engine's intake manifold to develop power. The vacuum booster is round steel container which is mounted on the firewall and is situated between the brake pedal and the master cylinder. Brake boosters produce line pressure in relation to the weight of the vehicle. Two types of boosters are available, the single and the dual diaphragm type.




Pricing
Single diaphragm brake boosters are generally less expensive than the dual diaphragm models. A 7-inch diameter single diaphragm brake booster is priced at $129.95 as of 2010 at Parker County Performance. A dual diaphragm model with the same diameter would cost $159.95 as of 2010. Larger diameter brake boosters whether single or dual diaphragm will yield higher prices.


Single Diaphragm Boosters
Single diaphragm power brake boosters can produce up to 950 p.s.i. They work well with four wheel drum systems or disc brake front/drum rear systems. These braking systems are lighter than the four disc brake system. These work best on older cars, because drum brakes were a common feature, at least on the rear axle of these cars.

Dual Diaphragm Boosters
The website of MBM Brake Boosters states that disc brake systems require 1000 p.s.i. to be applied to the wheels. Dual diaphragm brake boosters are need for four wheel disc system because they can produce a higher level of brake assist even if their physical size is smaller. Dual diaphragm brake boosters tend to give extra braking even when the vacuum from the intake manifold is low. Blue Oval news states that the Mercury Mariner is standardly equipped with dual diaphragm brake boosters for its power assist in its braking system. The vehicle has disc front and rear drum set up.

Booster Sizes
Power brake boosters require 16 inches of vacuum or more to function properly according to Sachs Rod Shop, and smaller brake boosters require more vacuum to perform satisfactorily. Smaller brake boosters may be sufficient for tight engine compartments, but they require a stronger vacuum to function properly.

Conclusion
Power brakes are not required to stop a vehicle with disc brakes, according to MBM Brake Boosters. Power brake boosters provide the driver with greater pedal feel and also deliver shorter stopping distances. Smaller diameter dual diaphragm brake boosters provide the stopping power of larger single diaphragm brake boosters. Mike Harington advises on the Super Chevy.com that one should choose braking parts carefully. Mismatched braking components can make the best parts perform poorly.

 

[vance.dykes]

did you check the master / booster push rod adjustment ?

 

[chtampa]

I have that booster on my 4 wheel disc car, no problems with my Master Power system. People never check the residual valves in the master cylinder. There shouldn't be any and most have no idea where they are or how to get them out. I bet the pump is not needed and he threw it trying to solve the problem.

 

[rdbird9]

Do you know if the previous owner changed that Booster? If not thats not for disc brakes. Its a drum drum booster and is a single diaphram you need a dual diaphram for disc. I may be way off others will chime in.

I looked through some old under-hood pictures of the car pre-conversion and it definitely has a different booster now though I'm not sure about the type.

 

[rdbird9]

did you check the master / booster push rod adjustment ?

No I did not. This was one of my ideas as well but I have not actually checked it yet. From what I have looked at some pushrods are adjustable and some are not. I have the factory shop manual but I would assume the factory pushrod free-play spec may be useless now because of the non-stock brake system.

 

[rdbird9]

I have that booster on my 4 wheel disc car, no problems with my Master Power system. People never check the residual valves in the master cylinder. There shouldn't be any and most have no idea where they are or how to get them out. I bet the pump is not needed and he threw it trying to solve the problem.

I hadn't thought of that but since I'm pretty sure this has a disc/drum master cylinder it should not have a residual pressure valve in it. I'm not positive on that though.

 

[BeepBeepRR]

Can you take a picture of your proportioning valve? I know disc/drum uses a 2 piece with a metering block for the front wheels. And an adjustable valve for the rear. Maybe its the same for a disc/disc setup.

 

[vance.dykes]

No I did not. This was one of my ideas as well but I have not actually checked it yet. From what I have looked at some pushrods are adjustable and some are not. I have the factory shop manual but I would assume the factory pushrod free-play spec may be useless now because of the non-stock brake system.

using the f s m is a starting point. the rod to auj is behind the master cylinder the one in the / on the brake peddle is not adjustable.

 

[chtampa]

I hadn't thought of that but since I'm pretty sure this has a disc/drum master cylinder it should not have a residual pressure valve in it. I'm not positive on that though.

Disc/Drum are what has the residual valve. Designed to keep pressure on one section of the brakes. If this is removed and the distribution block has the shuttle valve removed then you are able to use an adjustable manual proportion valve. The stock distribution valve is not designed for 4 wheel discs. The shuttle valve in it will move and transfer pressure to front or rear when you use the brakes. Multiple things are going on.
I found this out while working on my system. It fixed mine.

 

[rdbird9]

Can you take a picture of your proportioning valve? I know disc/drum uses a 2 piece with a metering block for the front wheels. And an adjustable valve for the rear. Maybe its the same for a disc/disc setup.

Here's a picture of the valve. It's really tucked down there so it's kind of hard to get a clear picture from the top down. It looks like one of the generic black proportioning valves from JEGS or SSBC. From what I've read, the adjustment knob is for the rears.

 

[rdbird9]

using the f s m is a starting point. the rod to auj is behind the master cylinder the one in the / on the brake peddle is not adjustable.

Thanks for the tip on the two rods. I looked in the manual this evening. It only gives a spec for the Midland Ross booster rod (not the booster I have) of .910"-.920" so I may try that as a reference point. I'll look at it this weekend.

 

[rdbird9]

Disc/Drum are what has the residual valve. Designed to keep pressure on one section of the brakes. If this is removed and the distribution block has the shuttle valve removed then you are able to use an adjustable manual proportion valve. The stock distribution valve is not designed for 4 wheel discs. The shuttle valve in it will move and transfer pressure to front or rear when you use the brakes. Multiple things are going on.
I found this out while working on my system. It fixed mine.

Okay so my master would have originally had a residual pressure valve in the front port (rears) that should have been or needs to be removed. I would tend to think it was removed because if it was still there wouldn't it be causing the rear disc brakes to drag/overheat? I think the aftermarket prop. valve also serves as the distribution block as there seems to be no other junction points in the system.

 

[chtampa]

Okay so my master would have originally had a residual pressure valve in the front port (rears) that should have been or needs to be removed. I would tend to think it was removed because if it was still there wouldn't it be causing the rear disc brakes to drag/overheat? I think the aftermarket prop. valve also serves as the distribution block as there seems to be no other junction points in the system.

It is a common issue when DB are installed. The shuttle moves when you press the pedal to equalize the pressure front to rear. The system does not separate the front and rear so you can't really visualize what is happening. When I isolated the front and rear it was easy to get the brakes to work. Perhaps someone else can explain it better than I can. That was the solution for me. It was difficult for me to grasp but I was instructed to just do it and it would be fine. It worked. This may or may not be your issue.

 

[biomedtechguy]

No intent to hijack, but I am having a complete Wilwood 6 piston 4 wheel disc brake system installed. From the Wilwood master cylinder to the discs, all new components including stainless steel brake lines. I chose to keep them manual brakes and the diameter of the master cylinder is sized accordingly. The work is underway, so I haven't got any reports to offer.
To me, power brakes are like a light switch, on or off with little feedback in pedal feel. I'm thinking that I'll have a little more pedal movement and a better idea of how much force is being applied without a booster in the mix.
Opinions?

 

[rdbird9]

So I checked my master cylinder pushrod length against the .920" spec in the manual and mine is already adjusted longer than that. The spec in the manual was for a different style booster though, but I thought maybe... So I adjusted the rod out 3/4 turn. My brother wanted to drive the car tonight and I just rode along so I can't really say if the brakes feel any better or not. I'll drive it sometime in the next couple of days and report back.

It is a common issue when DB are installed. The shuttle moves when you press the pedal to equalize the pressure front to rear. The system does not separate the front and rear so you can't really visualize what is happening. When I isolated the front and rear it was easy to get the brakes to work. Perhaps someone else can explain it better than I can. That was the solution for me. It was difficult for me to grasp but I was instructed to just do it and it would be fine. It worked. This may or may not be your issue.

I'll take another look under the car sometime and see if I can find this.

 

[rdbird9]

No intent to hijack, but I am having a complete Wilwood 6 piston 4 wheel disc brake system installed. From the Wilwood master cylinder to the discs, all new components including stainless steel brake lines. I chose to keep them manual brakes and the diameter of the master cylinder is sized accordingly. The work is underway, so I haven't got any reports to offer.
To me, power brakes are like a light switch, on or off with little feedback in pedal feel. I'm thinking that I'll have a little more pedal movement and a better idea of how much force is being applied without a booster in the mix.
Opinions?

The only non-power brakes I've driven are the manual 4-wheel drums on my '66 VW Bug. However, that is a far cry from the system you are installing. There definitely is a more mechanical-feeling connection with the VW's manual brakes than the other cars I've driven.