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Discussion Starter · #1 ·
All- I have a question. I recently purchased a Right Stuff brake conversion for my old 1964 Impala.

Frankly they are nice people but I've had one issue after the next with the kit and I will explain more later on that.

For now- the brakes are working with good pedal height and firm pedal too and it stops much better than with the manual old drum brakes. However, in a panic stop situation no matter how hard I push the pedal I can't make any of the wheels lock up or skid. I'm trying to break in the pads/rotors slowly but wouldn't you think I would be able to lock the brakes up?

I'm heading out now for a long rod test and will report more later.

Thanks in advantage for any thoughts and opinions!

Dave
 

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Discussion Starter · #3 ·
Pedal ratio- not aware, its suppose to work with my original pedal and rod location
Booster size- 8" dual diaphram seams to be working perfectly
Master cylinder bore- Corvette design 1.125" with conventional DOT 3 fluid
Proportional valve(or other F/R adjustment)- typical GM brass proportioning valve no adjustment
Caliper piston(s) bore(per wheel)- based on 1979 Eldorarado single piston cailpers per wheel
Pad material- semi metalic with drilled and slotted 11" rotors

Thanks!!!!
 

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I think your master is to large.

Not knowing the ratio is a shot in the dark. But a smaller master yeilds higher pressures and should provide better stopping power with more pedal travel as the trade off.

You may also be able to find a smaller bore caliper that still fits in the bracket to adjust the braking that way.

Personally I would rather change 1 master then see how it feels before reducing the caliper bore.
 

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Pedal ratio- not aware, its suppose to work with my original pedal and rod location
If that's the case, then you the only conversion from manual to power or power to manual that doesn't require relocation of the master cylinder clevis on the pedal.

I've never done a conversion a car like yours, but multiple 70's and 80's. And everyone of them requires a relocation of the master cylinder rod to achieve a different ratio for pedal effort.
 

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You have a very common problem with essentially a "one-size fits all" aftermarket kit that may physically fit OK, but is not well engineered. I saw similar problems with several 55-57 Chevy car power brake conversions. Some of those kits are total Chinese junk. I'd guess that maybe 20% of them actually worked well out of the box.

As mentioned above, using a smaller bore master cylinder and/or raising the pushrod on the brake pedal should help. But, IMO, a 7/8" bore seems too small for use with a booster, and could cause a touchy pedal. Your fixed prop valve probably reduces rear braking power by 25% or more, and may not even be needed. All these kits simply ASSume a prop valve will be needed, w/o taking into consideration front-to-rear weight bias, tire sizes, or even front and rear brake effectiveness.

A question: How much vacuum does your engine make at idle?
 

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Discussion Starter · #8 ·
If that's the case, then you the only conversion from manual to power or power to manual that doesn't require relocation of the master cylinder clevis on the pedal.

I've never done a conversion a car like yours, but multiple 70's and 80's. And everyone of them requires a relocation of the master cylinder rod to achieve a different ratio for pedal effort.
Just to follow-up- my car has two possible mounting postions for the booster rod.... the top hole closest to the pivot is for manual brakes and the lower is for power brakes. Mine is assembled in the lower mounting hole.
 

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Discussion Starter · #9 ·
You have a very common problem with essentially a "one-size fits all" aftermarket kit that may physically fit OK, but is not well engineered. I saw similar problems with several 55-57 Chevy car power brake conversions. Some of those kits are total Chinese junk. I'd guess that maybe 20% of them actually worked well out of the box.

As mentioned above, using a smaller bore master cylinder and/or raising the pushrod on the brake pedal should help. But, IMO, a 7/8" bore seems too small for use with a booster, and could cause a touchy pedal. Your fixed prop valve probably reduces rear braking power by 25% or more, and may not even be needed. All these kits simply ASSume a prop valve will be needed, w/o taking into consideration front-to-rear weight bias, tire sizes, or even front and rear brake effectiveness.

A question: How much vacuum does your engine make at idle?
My engine is making 12.5" of vacuum at idle and accroding to the tech man where I bought the system that could be the problem. He said the 8" dual booster at that vacuum may not be providing enough assist. So that being said I'm going to add a vacuum pump to test the system.
 

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Discussion Starter · #10 ·
All- thanks for the responses...at the direction of the tech man I'm going to add a vacuum pump as a test to make certain the booster has enough vacuum to perform correctly. If that dosen't change anything I will then reposition the booster push rod to the upper mounting postion on the pedal lever. If no improvement then we are going to test the hydraulic PSI at the calipers.

Also since there were some comments about generic kits. I looked at several kits and most all of them looked like the companies copy each other or have similar parts in the systems. Mine is comprised of China, India and US parts.

Money was not the driving factor in my decision as I also looked at Wilwood and Brembo, great products, as I've used them on other cars but most of their products would require me to change to larger wheels and I really didn't want to do that. I also didn't want the total aftermarket look either.

I will keep all posted as I move along with this problem.
 

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The engine might be making 12.5" at idle but it routinely spikes to 20" or more when driving. Pretty much any time the engine is braking then the vacuum is gonna spike and the booster should store that negative pressure and so you'll have more than 12.5 stored most of the time. I would think you would notice poor braking more after long periods of idling and creeping through traffic when the brake gets cycled more often but the engine vacuum stays about the same. In other words, typically vacuum problems manifest themselves as they work okay when normal driving but the pedal gets harder in traffic and spirited driving.
If the pedal height is okay, I'd try the hole closest to the pivot on the pedal because it's free and if you didn't like that then try a different pad because it's the easiest thing to do and if it still wasn't acceptable that go the 1" MC route.
Do you know the calipers bore size?
Keep in mind, Locking up tires depends entirely on the tires ability to grip. Big fat wide tires with more grip are gonna be a lot harder to lock up then small skinny hockey pucks. Ideally, the maximum performance would be a full pedal with tires screeching but still rolling. Ya gotta be able to stop and steer at the same time.
 

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Who’s to say the holes in the pedal are correct.
coulda been someone drilled a new one above the manual brake location by mistake when it should have been below.
Whip out the tape measure and check the ratios to be sure.
This^^.

How much travel does your M/C have? Are you using all of the travel or is the pedal geometry causing the pedal to be hard to push the last bit of travel.

And I'm with Johnsongrass, a booster pump is not going to resolve anything unless the brake is only hard to push after using the brakes for multiple pumps of the pedal. You mentioned an 8" double diaphram boost - why so small?
 

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You generally need heaps more vacuum when going from 2 wheel disc brakes to 4 wheel disc brakes.
Nobody tells you that when they sell this syuff to you.
You will need to find more vacuum and generally that will be provided by a quite expensive vacuum pump and storage system that can be purchased from many suppliers including Speedway Motors.
Lucky you...
 

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Just to follow-up- my car has two possible mounting postions for the booster rod.... the top hole closest to the pivot is for manual brakes and the lower is for power brakes. Mine is assembled in the lower mounting hole.
And it should be that way... with the factory power brake setup. It's possible that the conversion setup you have was designed to replace manual brakes and assumed the manual brake pedal ratio. Won't take but a few minutes to move it and check the results.

As someone noted, the designer probably just assumed a proportioning valve would be needed, but it may not. I'm an AMC enthusiast and did a lot of research on AMC factory brakes (dating from late 50s Ramblers to the last 83 RWD AMC car). Almost every US manufacturer used a proportioning valve in the late 60s through the 70s. Around 79-80 AMC dropped the rear proportioning valve (actually a combination valve used since 1970 -- has proportioning valve, brake light warning switch, junction block, and in the early 70s metering valve all in one). They simply balanced the brakes by altering the size of the rear drums and wheel cylinders (most often wheel cylinder).

In a nutshell, you may or may not need a proportioning valve. They aren't linear -- they don't just reduce pressure by 25%. They reduce pressure proportionately -- on light braking they don't reduce it at all, but on sudden hard braking they close and reduce it by the percentage they are set at. I tried to find a really good web page explaining how they work, but most that I found only explained why they were used, and a couple were plain WRONG! This is the best description of what's going on. Scroll down to "Valves FAQ". Master Power Brakes FAQs. Just note that not all combination valves have the metering function -- it was left out of most starting in the late 70s. AMC used combo valves with metering from 71-77 or so (I might be off +/- a year on the end date -- going from memory!), and without metering until 79-80 when the combo valves were dropped.

The way to determine if you need one is to first find a dirt or gravel road/parking lot, or a wet (but not standing water) paved parking lot -- with plenty of room! Run up to 45-50 mph and hit the brakes kind of hard. If the rears lock up before the fronts you need a proportioning (or combo) valve. If they don't lock at the first hard stop you're still not done! Try it several times, braking harder and harder until you're hitting the brakes as hard and fast as you can at 45-50 mph. If the rears still don't lock first you're good to go! Yeah, you could just do that panic stop at first, but you don't know what the car is going to do. The rear will usually start to swing out to one side, and if the brake balance is real bad you could end up spinning out the first time. Don't want that to happen!

A proportioning valve usually won't hurt brake performance even if you don't really need it, so the aftermarket guys include it. As noted, on light braking it doesn't do anything, and on a panic stop the fronts are going to be doing 60-70 percent of the work, and the rears will still be helping you stop even if pressure is reduced. If you can't lock the brakes at all you may not need one, but just remember that locked brakes aren't doing much to slow a car -- the rubber is sliding, and hot rubber slides easy. That's why us old guys were taught to "pump the brakes" if the wheels locked, which is exactly what ABS brakes do (only really fast!).
 
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Discussion Starter · #18 ·
All- I appreciate all your input. At the request of the tech guy, I ordered and installed a vacuum pump designed for brake booster applications.

First I used my electric pump and it did allow the pedal to drop about another inch so it did push the rod deeper into the master cylinder. So one would think it was creating more pressure on the hydraulics but unfortunately it changed nothing as far as stopping the car.

I really think it's the master cylinder bore size. So next I'm going to test the pressure at each caliper.

With the vacuum pump running the assist is so great that I can hold my foot on the pedal and push it to the floor. I assume it's bypassing the plunger seals???? OR IS THIS AN INDICATION THAT THE MASTER CYLINDER BORE IS TO BIG?

Thanks
 

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Sorry to read that you are still feeling the pain with this issue.
I always like working with 1" bore size if possible.
I guess once your brake are worked out, at least you will have the best vacuum possible to get those brakes working good.
Keep us posted.
I installed an 1" bore m/c in my old F100 and discovered pedal ration issues, but it does stop and it can lock up the brakes, just had to get used to the additional travel in the pedal.
Now I am used to it I like it.

Vacuum boosted brakes like you will end up with can be deadly on wet roads so keep an eye on that.
For me IMHO, I prefer to save my money and stick with rear drum brakes.
They have been around fro a long time are OK for my needs.

Some of those after market small diameter single diaphram boosters you see for cheap are junk and my advice is avoid them if possible...
 

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Could be the bore of the MC. As noted, a smaller bore increases pressure, larger bore reduces. In general the larger bore will require less pedal stroke as it moves more fluid per inch of travel -- a smaller bore requires more stroke to move the same amount of fluid. All depends on the caliper volume required. C3 Corvettes use a large 1.125" bore MC, but have rather large four piston calipers up front, C4 and later used bores from 0.875" -0.930".... close to 1". They have a bit smaller caliper.
 
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